NCERT Solutions Class 8 Science

Technology is advancing at a fast pace and it’s important to keep pace with the changing technology.  Science helps the students to develop life skills, observe, understand the natural world and ask questions. It is necessary to understand the concepts of science at an early stage. The Central Board of Secondary Education has taken various measures to help the students develop their technical skills to achieve success in this field.

The NCERT Solutions for Class 8 Science are exclusive and vital tools for Science. Experts in Science have developed them with their years of experience. To access NCERT Science book class 8 solutions, you can check out the  Extramarks website.

NCERT Solutions for Class 8 Science | Chapter Wise

The NCERT Science book class 8 solutions have been provided below:

Chapter 1: Crop Production and Management

Chapter 2: Microorganisms: Friend and Foe

Chapter 3: Synthetic Fibres and Plastics

Chapter 4: Materials – Metals and Non-Metals

Chapter 5: Coal and Petroleum

Chapter 6: Combustion and Flame

Chapter 7: Conservation of Plants and Animals

Chapter 8: Cell – Structure and Functions

Chapter 9: Reproduction in Animals

Chapter 10: Reaching the Age of Adolescence

Chapter 11: Force and Pressure

Chapter 12: Friction

Chapter 13: Sound

Chapter 14: Chemical Effects of Electric Current

Chapter 15: Some Natural Phenomena

Chapter 16: Light

Chapter 17: Stars and The Solar System

Chapter 18: Pollution of Air and Water

NCERT Class 8 Science Solutions –  Your Ultimate Guide & Mentor

Middle school is a crucial phase for students who wish to pursue science in higher classes, and thus, it becomes imperative to grasp the fundamental concepts of science at this level.  Instead of racking your brain for the right mentor, just imagine getting all the questions and answers of your textbook at one place, at your doorstep. How cool would that be? On the official website of Extramarks, you can get all the NCERT solutions for class 8th Science which will help you prepare for the science examination. Given below are the core contents of each chapter of Class 8 Science.

NCERT Solutions for Class 8 Science Chapter 1 Crop Production and Management

Chapter 1: Crop Production and Management

1.1: Agricultural Practices

1.2: Basic Practices of Crop Production

1.3: Preparation of Soil

1.4: Sowing

1.5 Adding Manure and Fertilisers

1.6: Irrigation

1.7: Protection from Weeds

1.8: Harvesting

1.9: Storage

1.10: Food from Animals

NCERT Solutions for Class 8 Science Chapter 2 Microorganisms: Friend and Foe

Chapter 2: Microorganisms: Friend and Foe

2.1: Microorganisms

2.2: Where do Microorganisms Live?

2.3: Microorganisms and Us

2.4: Harmful Microorganisms

2.5: Food Preservation

2.6: Nitrogen Fixation

2.7: Nitrogen cycle

NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fibres and Plastics

Chapter 3: Synthetic Fibres and Plastics

3.1: What are Synthetic Fibres?

3.2: Types of Synthetic Fibres

3.3: Characteristics of Synthetic Fibres

3.4: Plastics

3.5: Plastics as Materials of Choice

3.6: Plastics and the Environment

NCERT Solutions for Class 8 Science Chapter 4 Materials: Metals and Non-Metals

Chapter 4: Materials: Metals and Non-Metals

4.1: Physical Properties of Metals and Non-metals

4.2: Chemical Properties of Metals and Non-metals

4.3: Uses of Metals and Non-metals

NCERT Solutions for Class 8 Science Chapter 5 Coal and Petroleum

Chapter 5: Coal and Petroleum

5.1: Coal

5.2: Petroleum

5.3: Natural Gas

5.4: Some Natural Resources are Limited

NCERT Solutions for Class 8 Science Chapter 6 Combustion and Flame

Chapter 6: Combustion and Flame

6.1: What is Combustion?

6.2: How Do We Control Fire?

6.3Types of Combustion

6.4: Flame

6.5: Structure of a Flame

6.6: What is a Fuel?

6.7: Fuel Efficiency

NCERT Solutions for Class 8 Science Chapter 7 Conservation of Plants and Animals

Chapter 7: Conservation of Plants and Animals

7.1: Deforestation and Its Causes

7.2: Consequences of Deforestation

7.3: Conservation of Forest and Wildlife

7.4: Biosphere Reserve

7.5: Flora and Fauna

7.6: Endemic Species

7.7: Wildlife Sanctuary

7.8: National Park

7.9: Red Data Book

7.10: Migration

7.11: Recycling of Paper

7.12: Reforestation

NCERT Solutions for Class 8 Science Chapter 8 Cell: Structure and Functions

Chapter 8: Cell – Structure and Functions

8.1: Discovery of the Cell

8.2: The Cell

8.3: Organisms show Variety in Cell Number, Shape and Size

8.4: Cell Structure and Function

8.5: Parts of the Cell

8.6: Comparison of Plants and Animals Cells

NCERT Solutions for Class 8 Science Chapter 9 Reproduction in Animals

Chapter 9: Reproduction in Animals

9.1: Modes of Reproduction

9.2: Sexual Reproduction

9.3: Asexual Reproduction

NCERT Solutions for Class 8 Science Chapter 10 Reaching the Age of Adolescence

Chapter 10: Reaching The Age of Adolescence

10.1: Adolescence and Puberty

10.2: Changes at Puberty

10.3: Secondary Sexual Characters

10.4: Role of Hormones in Initiating Reproductive Function

10.5: Reproductive Phase of Life in Humans

10.6: How is the Sex of the Baby Determined?

10.7: Hormones other than Sex Hormones

10.8: Role of Hormones in Completing the Life History of Insects and Frogs

10.9: Reproductive Health

NCERT Solutions for Class 8 Science Chapter 11 Force and Pressure

Chapter 11: Force And Pressure

11.1: Force: A push or a Pull

11.2: Forces are due to an Interaction

11.3: Exploring Forces

11.4: A Force can Change the State of Motion

11.5: Force can Change the Shape of an object

11.6: Contact Forces

11.7: Non-contact Forces

11.8: Pressure

11.9: Pressure Exerted by Liquids and Gases

11.10: Atmospheric Pressure

NCERT Solutions for Class 8 Science Chapter 12 Friction

Chapter 12: Friction

12.1: Force of  Friction

12.2: Factors affecting Friction

12.3: Friction: A Necessary Evil

12.4: Increasing and Reducing Friction

12.5: Wheels Reduce Friction

12.6: Fluid Friction

NCERT Solutions for Class 8 Science Chapter 13 Sound

Chapter 13: Sound

13.1: Sound is Produced by Vibrating Bodies

13.2: Sound Produced by Humans

13.3: Sounds Needs a Medium for Propagation

13.4: We Hear Sound through Our Ears

13.5: Aptitude, Time Period and Frequency of a vibration

13.6: Audible and Inaudible Sounds

13.7: Noise and Music

13.8: Noise Pollution

NCERT Solutions for Class 8 Science Chapter 14 Chemical Effects of Electric Current

Chapter 14: Chemical Effects Of Electric Current

14.1: Do Liquids Conduct Electricity?

14.2: Chemical Effects Of Electric Current

14.3: Electroplating

NCERT Solutions for Class 8 Science chapter 15 Some Natural Phenomena

Chapter 15: Some Natural Phenomena

15.1: Lightning

15.2: Charging by Rubbing

15.3: Types of Charges and Their Interaction

15.4: Transfer of Charge

15.5: The Story of Lightning

15.6: Lightning Safety

15.7: Earthquakes

NCERT Solutions for Class 8 Science Chapter 16 Light

Chapter 16: Light

16.1: What makes Things Visible

16.2: Laws of Reflection

16.3: Regular and Diffused Reflection

16.4: Reflected Light Can be Reflected Again

16.5: Multiple Images

16.6: Sunlight – White or Coloured

16.7: What is inside Our Eyes?

16.8: Care of the Eyes

16.9: Visually Impaired Persons Can Read and Write

16.10: What is the Braille System?

NCERT Solutions for Class 8 Science Chapter 17 Stars and The Solar System

Chapter 17: Stars And The Solar System

17.1: The Moon

17.2: The Stars

17.3: Constellations

17.4: The Solar System

17.5: Some Other Members of the Solar System

NCERT Solutions for Class 8 Science Chapter 18 Pollution of Air and Water

Chapter 18: Pollution of Air and Water

18.1: Air Pollution

18.2: How does Air Get Polluted?

18.3: Case Study- The Taj Mahal

18.4: Greenhouse Effect

18.5: What can be done?

18.6: Water Pollution

18.7: How does Water Get Polluted?

18.8: What is Potable Water and How is Water Purified?

18.9: What Can be Done?

Why is Extramarks the Best Class 8 Science Solution Provider?

Extramarks give you the competitive edge and help you comprehend the scientific concepts by providing the best NCERT Science class 8 solutions. After a crucial analysis of the chapters, the subject matter experts have prepared these solutions to give the students 100% authentic and complete guidance.  Besides this, it also offers NCERT solutions class 11, & NCERT solutions class12  for all the subjects in every stream.

Extramarks focuses on providing complete learning solutions, and the same goes for NCERT science class 8. We have updated answers according to the latest CBSE class 8 science syllabus, which will also prepare the students to crack competitive exams JEE Main/Advanced, VITEEE,  and other exams.

Why are NCERT Science Solutions important for Class 8 students?

Middle school Science comprises Physics, Chemistry, and Biology and often gets pretty tough to understand because CBSE, through its NCERT curriculum, has set a high standard. With working parents and the absence of proper guidance, in this scenario, NCERT solutions for class 8th science become a vital tool that comes in handy to the students of class 8. 

Being an average student, is it possible to get a high score in the CBSE Class 8 Science exam?

Some tips which can be followed to score well in CBSE class 8 Science question paper are given below:

  • Class 8 Science requires lots of practice and learning from day one, so create a proper study plan and stick to it.
  • Solve as many sample papers as possible. Previous year questions can also be used for further practice.
  • Follow the Solutions of NCERT Science class 8, which you can find on the official website of Extramarks
  • Revise thoroughly and take frequent self-tests to assess yourself.
  • Practicing the diagrams repeatedly will improve your score and confidence level.
  • NCERT Solutions can be your best guide and mentor to boost your performance in all subjects not just Science. 

Why should you refer to Class 8 Science NCERT Solutions? How is it different?

Some of the benefits of referring to class 8 Science NCERT solutions are given below:

  • The answers provided are concise and to the point
  • The Subject matter experts with years of teaching experience have updated and cross-checked the solutions.
  • The solutions have been simplified without any repetition, making it easier for the students to understand and grasp the concepts quickly and easily.
  • The solutions help the students in cracking the examination with excellent results. 
  •  The  NCERT Science Solutions can be found at the Extramarks website to help the students download Solutions free of cost and make the most of it.

Salient Features of Extramarks’ NCERT Science Solutions Class 8

Some salient features of Extramarks NCERT Science Solutions Class 8 are given below:

  • It assists the students of class 8 to prepare well for their science examination. It is a suitable revision material
  • It provides in-depth knowledge of the subject to increase your confidence level while writing your exam
  • It is a quick resource for learning written in simple English to ensure that all the students can understand it
  • It helps quickly grasp all the ideas and concepts related to Physics, Chemistry, and Biology
  • The NCERT Solutions by Extramarks strives to reach out to each and every learner right from the foundation level to the Secondary level to excel in life. 

Q.1 Explain the importance of reproduction in organisms.

Ans

Importance of reproduction in organisms is:

  1. Reproduction ensures the continuity of a species.
  2. It also maintains the number of individuals in a population.
  3. It is also responsible for variations in organisms in a population that help them to survive in the environment.

Q.2 Describe the process of fertilisation in human beings.

Ans

Fusion of a sperm and an ovum is called fertilisation. In human beings, the process of fertilisation takes place in the oviducts. The sperms produced by testes in male’s body are transferred into the female body. In female’s body, one matured ovum is released into the oviduct by one of the ovaries every month. In the oviduct, the sperm comes in contact with the ovum and fuses with it. During the process of fusion, the nuclei of the sperm and the ovum fuse to form one single nucleus. This results in the formation of zygote.

Q.3 Choose the most appropriate answer.
a) Internal fertilisation occurs
(i) in female body.
(ii) outside female body.
(iii) in male body.
(iv) outside male body.

(b) A tadpole develops into an adult frog by the process of
(i) fertilisation
(ii) metamorphosis
(iii) embedding
(iv) budding

(c) The number of nuclei present in a zygote is
(i) none
(ii) one
(iii) two
(iv) four

Ans

(a) (i) in female body.

(b) (ii) metamorphosis

(c) (ii) one

Q.4 Indicate whether the following statements are True (T) or False (F).
(a) Oviparous animals give birth to young ones. ( )
(b) Each sperm is a single cell. ( )
(c) External fertilisation takes place in frog. ( )
(d) A new human individual develops from a cell called gamete. ( )
(e) Egg laid after fertilisation is made up of a single cell. ( )
(f) Amoeba reproduces by budding. ( )
(g) Fertilisation is necessary even in asexual reproduction. ( )
(h) Binary fission is a method of asexual reproduction. ( )
(i) A zygote is formed as a result of fertilisation. ( )
(j) An embryo is made up of a single cell. ( )

Ans

(a) Oviparous animals give birth to young ones. (F)
(b) Each sperm is a single cell. (T)
(c) External fertilisation takes place in frog. (T)
(d) A new human individual develops from a cell called gamete. (F)
(e) Egg laid after fertilisation is made up of a single cell. (T)
(f) Amoeba reproduces by budding. (F)
(g) Fertilisation is necessary even in asexual reproduction. (F)
(h) Binary fission is a method of asexual reproduction. (T)
(i) A zygote is formed as a result of fertilisation. (T)
(j) An embryo is made up of a single cell. (F)

Q.5 Give two differences between a zygote and a foetus.

Ans

Differences between a zygote and a foetus are:

Zygote Foetus
It is unicellular. Hence, no well-defined body parts. It is multicellular and has well-defined body parts.
It forms due to the fertilisation of sperm and ovum. It forms after the process of fertilisation in which zygote multiplies and grows into a multicellular body.

Q.6 Define asexual reproduction. Describe two methods of asexual reproduction in animals.

Ans

Asexual reproduction is a type of reproduction in which only one parent is involved and the offspring is genetically identical to the parent cell.

Two methods of asexual reproduction are budding and binary fission.

  1. Budding: In this type of asexual reproduction, an outgrowth appears on the parent’s body. This outgrowth is termed as bud. The bud gradually grows into a small individual and then detaches itself from the parent’s body. It continues to grow and live as a new individual in the environment. Eg; Hydra.
  2. Binary fission: In this type of asexual reproduction, the nucleus of the parent’s body divides into two nuclei. After that the body divides into two and each body part receives a nucleus. Eg; Amoeba.

Q.7 In which female reproductive organ does the embryo get embedded?

Ans

The embryo gets embedded in the walls of uterus. In the uterus, the embryo grows and develops all the body parts.

Q.8 What is metamorphosis? Give examples.

Ans

Metamorphosis is a biological process of transformation of an immature form, such as larva, into an adult through drastic changes. It is observed in frogs (transformation of tadpole into adult frog) and in insects (transformation of larva into moths or butterflies).

Q.9 Differentiate between internal fertlisation and external fertlisation.

Ans

Characters Internal Fertilisation External Fertilisation
Site of fertilisation Inside female body Outside female body
Number of eggs/offspring produced Less Large
Chances of survival of eggs/offspring High Low
Example Mostly mammals Generally fish and frog

Q.10 Complete the crossword puzzle using the hints given below.
Across
1. The process of the fusion of the gametes.
6. The type of fertilisation in hen.
7. The term used for bulges observed on the sides of the body of hydra.
8. Eggs are produced here.

Down
2. Sperms are produced in these male reproductive organs.
3. Another term for in vitro fertilisation.
4. These animals lay eggs.
5. A type of fission in amoeba.

Ans

Q.11 Select the correct word from the following list and fill in the blanks.
float, water, crop, nutrients, preparation
(a) The same kind of plants grown and cultivated on a large scale at a place is called _______.
(b) The first step before growing crops is _____ of the soil.
(c) Damaged seeds would ______ on top of water.
(d) For growing a crop, sufficient sunlight and _____ and________ from the soil are essential.

Ans

(a) Crop
(b) preparation
(c) float
(d) water, nutrients

Q.12 Match items in column A with those in column B.

A

B

(i) Kharif crops (a) Food for cattle
(ii) Rabi crops (b) Urea and super phosphate
(iii) Chemical fertlilisers (c) Animal excreta, cow dung, urine and plant waste
(iv) Organic manure (d) Wheat, gram, pea
(e) Paddy and maize

Ans

A

B

(i) Kharif crops (e) Paddy and maize
(ii) Rabi crops (d) Wheat, gram, pea
(iii) Chemical fertlilisers (b) Urea and super phosphate
(iv) Organic manure (c) Animal excreta, cow dung, urine and plant waste

Q.13 Give two examples of each.
(a) Kharif crop
(b) Rabi crop

Ans

(a) Two examples of Kharif crop are paddy and maize.
(b) Two examples of Rabi crop are wheat and gram.

Q.14 Write a paragraph in your own words on each of the following.
(a) Preparation of soil
(b) Sowing
(c) Weeding
(d) Threshing

Ans

(a) Preparation of soil: Before growing a crop, a farmer first prepares the soil of the field. During preparation, he turns the soil with the help of a plough or a cultivator and loosens it. Loosening of soil allows the new germinating roots of the plants to penetrate deep into the soil. The loose soil also facilitates easy breathing to the roots even when they go deep into the soil. The loosened soil helps in the growth of earthworms and microbes present in the soil which further turn and loosen the soil and add humus to it. Turning and loosening of soil brings the nutrient-rich soil to the top so that plants can use these nutrients.

(b) Sowing: After soil preparation, the farmer’s next job is to sow the seeds. First, good quality and high yielding seeds are selected. Seeds are sown with the help of a tool called seed drill. This tool facilitates the sowing of seeds uniformly at proper distances and depths and saves time and labour. It ensures that seeds get covered by the soil after sowing. This prevents damage caused by birds.

(c) Weeding: The undesirable plants that grow naturally along with the main crop planted are called weeds. These weeds compete with the main crop plant for water, nutrients, sunshine and space. Weeds can be controlled by weeding. Weeding is the removal of weeds from the field. Weeding can be done by tilling and by using weedicides. Some weeds interfere even in harvesting and may be poisonous for animals and human beings.

(d) Threshing: After harvesting a crop, its grain seeds need to be separated from the chaff. This process of separation of grain seeds from chaff is called threshing. This is done with the help of a machine called combine which is in fact a combined harvester and a thresher.

Q.15 Explain how fertlisers are different from manure.

Ans

Fertiliser Manure
It is a chemical substance which is rich in particular nutrients. It is natural substance aquired by the decompostion of organic waste such as cattle dung, human waste and plant residues.
It is decreases the soil fertility. It is increase the fertility of soil by replenishing the nutrients.
It does not provide any humus to the soil. It provides a lot of humus to the soil.
It is very rich in nutrients like nitrogen, phosphorus and pottasium. It is relatively less rich in plant nutrients.
It is responsible for soil and water pollution. It doesnot cause soil and water pollution.

Q.16 What is irrigation? Describe two methods of irrigation which conserve water.

Ans

Irrigation is a process by which water is supplied to crops. The time and frequency of irrigation depends on the seasons, crops and soil types.

Two methods of irrigation that help in conservation of water are:

  • Sprinkler system: In this system, perpendicular pipes with rotating nozzles on top are joined to the main pipe at regular intervals. With the help of a pump, water is allowed to flow through main pipe under high pressure. From the main pipes, the water escapes outside through the rotating nozzles. This system works efficiently in sandy soil where sufficient water is not available.
  • Drip irrigation: In this system, water is dropped at the root of the plants, drop by drop. Water is not wasted in this type of irrigation and this method is used for watering fruit plants and gardens.

Q.17 If wheat is sown in the kharif season, what would happen? Discuss.

Ans

If wheat is sown in the kharif season (from June to October), it has to face adverse climatic conditions and the entire wheat crop might get destroyed. Wheat is a rabi crop. Therefore, it requires low temperature, less humidity, less water and moderate sunshine. But if it is sown in kharif season, it would receive lot of water, excess temperature, and humidity. These climatic conditions would not favour the growth of the crop.

Q.18 Explain how soil gets affected by the continuous plantation of crops in a field.

Ans

Continuous plantation of crops in a field causes depletion of certain nutrients like phosphorus, nitrogen and potassium etc. Plants are depended on these nutrients for their proper growth and functioning. Depletion of these nutrients from the soil would directly affect the yield of the crops sown in the depleted soil.

Q.19 What are weeds? How can we control them?

Ans

The undesirable plants that grow naturally along with the main crop are called weeds. These weeds compete with the main crop plant for water, nutrients, sunshine and space. Weeds can be controlled by weeding. Weeding is the removal of weeds from the field.

Weeding can be done by:

Tilling: In this method, the weeds are either manually uprooted or cut from soil. The uprooted and cut weeds are then left in the field so that they would decay and act as the manure.

Using Weedicides: In this method, certain chemicals called weedicides such as 2, 4-D are sprayed over the fields to kill the weeds. Weedicides are used during the vegetative growth of weeds.

Q.20 Arrange the following boxes in proper order to make a flow chart of sugarcane crop production

Ans

Q.21 Complete the following word puzzle with the help of clues given below.
Down
1. Providing water to the crops.
2. Keeping crop grains for a long time under proper conditions.
5. Certain plants of the same kind grown on a large scale.

Across
3. A machine used for cutting the matured crop.
4. A rabi crop that is also one of the pulses.
6. A process of separating the grain from chaff.

Ans

Q.22 Fill in the blanks.
(a) A place where animals are protected in their natural habitat is called _______.
(b) Species found only in a particular area is known as ______.
(c) Migratory birds fly to far away places because of ______ changes.

Ans

(a) A place where animals are protected in their natural habitat is called wildlife sanctuaries.
(b) Species found only in a particular area is known as endemic.
(c) Migratory birds fly to far away places because of climatic changes.

Q.23 Differentiate between the following.
(a) Wildlife sanctuary and biosphere reserve
(b) Zoo and wildlife sanctuary
(c) Endangered and extinct species
(d) Flora and fauna

Ans

(a)

Wildlife Sanctuary Biosphere Reserve
An area within which animals are protected from possible dangers, such as hunting Large protected area constructed for the conservation of biodiversity
Provides protection and suitable living conditions to wild animals Helps in the conservation of various life forms, such as plants, animals and micro–organisms

(b)

Zoo Wildlife Sanctuary
A facility in which animals are kept for public presentation An area within which animals are protected from possible dangers, such as hunting
An artificial habitat Conserves the natural habitat of animals

(c)

Endangered species Extinct species
Population of species that is on the verge of becoming extinct Population of species that no longer exists. Hence, it is extinct
Blue whale, tiger, leopard etc. are examples of endangered species. Dodo, passenger pigeon etc. are examples of extinct species.

(d)

Flora Fauna
The term refers to all living plants in a particular area. The term refers to all animals living in a particular area.

Q.24 Discuss the effects of deforestation on the following.
(a) Wild animals
(b) Environment
(c) Villages (Rural areas)
(d) Cities (Urban areas)
(e) Earth
(f) The next generation

Ans

(a) Wild animals: Deforestation is the removal of trees or other vegetation from an area for industrial, agricultural and other purposes. Trees and other vegetation form the habitat of many animals. Hence, deforestation leads to the destruction of the habitats of wild animals, which in turn decline their population and may become the cause of their extinction.

(b) Environment: Deforestation increases the level of carbon dioxide in the atmosphere, as there would be less number of plants that absorb carbon dioxide from the atmosphere to perform photosynthesis. The increased level of carbon dioxide in the atmosphere will trap more heat radiations, thereby leading to global warming. Naturally, the temperature of the earth will increase. An increase in the temperature of the earth will disturb the natural water cycle, resulting in the change of rainfall pattern. This could lead to floods or draughts.

(c) Villages (Rural areas): Deforestation may result in less rainfall or heavy rains. Less rainfall will lead water scarcity in villages that will decrease the agricultural production. Hence, domestic cattle will have no fodder. Heavy rains will cause floods. Soil erosion caused by deforestation and floods will further adversely affect the agriculture, which will lead to migration of village people to the cities for their livelihood.

(d) Cities (Urban areas): As in the case of villages, deforestation in cities can increase the risk of floods and draughts in that area. In addition to that, vehicular and industrial pollution, increase in the level of carbon dioxide in the atmosphere, lead to global warming.

(e) Earth: Deforestation increases desertification, droughts, floods, etc. Due to deforestation, there will an increases in the temperature of the earth i.e., global warming due to the increased level of carbon dioxide on the earth. As a result, the entire natural water cycle will get disrupted and will increase the threat of natural calamities.

(f) The next generation: Deforestation is responsible for global warming, soil erosion, greenhouse effect, drought, floods and many other global problems. Due to this, our next generation will have to face severe consequences.

Q.25 What will happen if.
(a) we go on cutting trees
(b) the habitat of an animal is disturbed
(c) the top layer of soil is exposed

Ans

(a) Continuous cutting of trees will completely destroy habitat of many organisms. Cutting of trees will lead to global warming. Global warming will disturb the natural water cycle. As a result, rainfall pattern will be changed, which will further lead to floods or droughts. This will also increase the risk of soil erosion, desertification and natural calamities.

(b) The habitat is a place where an organism gets food, shelter and protection. If the habitat of an animal is disturbed, the animal will be forced to move to other places in search of food and shelter. In this process, the animal might get killed by other animals or unfavourable environmental conditions.

(c) If the top layer of the soil is exposed, it will gradually erode exposing the lower layer of soil, which is less fertile as it contains less humus. Continued soil erosion will make the land barren or infertile.

Q.26 Answer in brief.
(a) Why should we conserve biodiversity?
(b) Protected forests are also not completely safe for wild animals. Why?
(c) Some tribals depend on the jungle. How?
(d) What are the causes and consequences of deforestation?
(e) What is Red Data Book?
(f) What do you understand by term migration?

Ans

(a) Biodiversity should be conserved to maintain a balance in nature, as each component of biodiversity is dependent on each other for their sustainability.
(b) Protected forests are also not completely safe for wild animals because people living in villages adjacent to the forest use forest resources and kill animals for their flesh, skin, etc.
(c) Tribals are the people that live in forests and are away from the modern life amenities. For their survival, forest is the only source that provides them food items like, grains, fruits, and other products like meat, honey, fodder for their cattle, wood fuel etc.
(d) Causes of deforestation: To meet the demands of increasing human population, urbanisation and industrialisation
Consequences of deforestation: Deforestation will lead to global warming, environmental imbalances like disturbed water cycle.
(e) Red Data Book is the source book which maintains a record of all the endangered animals and plants.
(f) Migration is the phenomenon of movement of a species from its habitat to other habitat for a particular time period in a year for a specific purpose, like breeding.

Q.27 In order to meet the ever-increasing demand in factories and for shelter, trees are being continually cut. Is it justified to cut trees for such projects? Discuss and prepare a brief report.

Ans

No, it is not at all justified to cut trees to meet the ever increasing demands of human population. Forests are the habitats of several organisms, including wild animals. They help in maintaining a balance between oxygen and carbon dioxide in nature and hence, keep a check on the temperature of the earth. In the process, they prevent the excessive heating of the atmosphere. They prevent soil erosion to a large extent and natural calamities, such as floods and droughts. They increase the fertility of the soil and help in conserving biodiversity. The cutting of forests to large extent will cause global warming, soil erosion, greenhouse effect, droughts, floods, and many more problems. Hence, forests must be conserved to maintain a balance in nature.

Q.28 How can you contribute to the maintenance of green wealth of your locality? Make a list of actions taken by you.

Ans

My contribution in the maintenance of green wealth of my locality will be:

  1. By taking care of the plants and trees growing in or around my locality
  2. By planting more and more trees
  3. By encouraging people of my locality to plant more trees
  4. By informing people about the importance of growing trees
  5. By educating people and young children about the effects of deforestation on our environment and on our planet

Q.29 Explain how deforestation leads to reduced rainfall.

Ans

Deforestation causes an increase in the level of carbon dioxide. The high levels of carbon dioxide in the atmosphere will trap more heat radiations, causing global warming. Due to an increase in the temperature of the earth, water cycle will get disturbed. As a result of this, the rainfall pattern will be changed. The reduced amount of rain will then cause droughts.

Q.30 Find out about national parks in your state. Identify and show their location on the outline map of India.

Ans

I live in New Delhi. There is no national park in New Delhi. One of the national parks located near Delhi is the Jim Corbett National Park.

Q.31 Why should paper be saved? Prepare a list of ways by which you can save paper.

Ans

Paper should be saved because it takes around seventeen full grown trees to make 100 kg of paper. Trees help in maintaining a balance of nature. Therefore, saving paper is a way to save trees and prevent the impact of their loss on living organisms.

Ways by which paper can be saved are as follows:

  1. Recycling of used paper.
  2. Writing on both the sides of a paper
  3. Educating people about the importance of paper
  4. Using of information technology to save printed data in digital form

Q.32 Complete the word puzzle.
Down
1. Species on the verge of extinction.
2. A book carrying information about endangered species.
5. Consequence of deforestation.

Across
1. Species which have vanished.
3. Species found only in a particular habitat.
4. Variety of plants, animals and microorganisms found in an area.

Ans

Q.33 Explain why some fibres are called synthetic.

Ans

Man-made fibres are called synthetic fibres. These fibres are prepared from petrochemicals with the help of various processes. Examples of some synthetic fibres are nylon, rayon, polyester, acrylic, etc.

Q.34 Mark (✓) the correct answer.

Rayon is different from synthetic fibres because

    1. it has a silk-like appearance.
    2. it is obtained from wood pulp.
    3. its fibres can also be woven like those of natural fibres.

Ans

Rayon is different from synthetic fibres because

  1. it has a silk-like appearance.
  2. it is obtained from wood pulp. (✓)
  3. its fibres can also be woven like those of natural fibres.

Q.35 Fill in the blanks with appropriate words.

    1. Synthetic fibres are also called ______ or ______ fibres.
    2. Synthetic fibres are synthesised from raw materials called _________.
    3. Like synthetic fibres, plastic is also a ________.

Ans

  1. artificial, man-made
  2. petrochemicals
  3. polymer

Q.36 Give examples which indicate that nylon fibres are very strong.

Ans

Nylon fibres are strong, elastic and light. In fact they are stronger than steel wire. They are used in making ropes for rock climbing and parachutes. This usage indicates that nylon fibres are very strong.

Q.37 Explain why plastic containers are favoured for storing food.

Ans

The plastic containers are favoured for storing food because plastic is non-reactive, corrosion resistant, cheap, durable, strong and light in weight.

Q.38 Explain the difference between thermoplastic and thermosetting plastics.

Ans

Thermoplastics Thermosetting Plastics
Thermoplastics can be softened and reshaped when heated, but harden again when cooled. Thermosetting plastics do not soften on heating. Hence, they can be moulded only once.
Thermoplastics can be bent easily. Thermosetting plastics cannot be bent easily and may break when forced to bend.
Polythene and PVC are some examples of thermoplastics. Bakelite and melamine are some examples of thermosetting plastics.
Thermoplastics are used for manufacturing toys, combs and various types of containers, etc. Thermosetting plastics are used for making electrical switches, handles of various utensils, etc.

Q.39 Explain why the following are made of thermosetting plastics.

    1. Saucepan handles
    2. Electric plugs/switches/plug boards

Ans

  1. Saucepan handles are made of thermosetting plastics because these plastics do not soften on heating.
  2. Thermosetting plastics are poor conductors of heat and electricity. Therefore, they are used in making electric plugs, switches, plug boards, etc.

Q.40 Categorise the materials of the following products into ‘can be recycled’ and ‘cannot be recycled’.

Telephone instruments, plastic toys, cooker handles, carry bags, ball point pens, plastic bowls, plastic covering on electrical wires, plastic chairs, electrical switches.

Ans

Can be recycled Cannot be recycled
plastic toys telephone instruments
carry bags cooker handles
ball point pens electrical switches
plastic bowls
plastic covering on electrical wires
plastic chairs

Q.41 Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made from synthetic material? Advise Rana, giving your reason.

Ans

Rana should buy cotton shirts. This is because cotton is a good absorber of water. Therefore, it absorbs the sweat (mainly water having dissolved salts) and exposes it to the environment. With the help of atmospheric heat, water present in sweat is evaporated. During evaporation water is lost and this water takes away heat with it. Thus, it helps in evaporating the water (sweat), thereby cooling our body.

Q.42 Give examples to show that plastics are noncorrosive in nature.

Ans

Plastics are non-reactive and noncorrosive in nature. They are not corroded even by strong chemicals. That is why various kind of chemicals are stored in plastics bottles.

Q.43 Should the handle and bristles of a tooth brush be made of the same material? Explain your answer.

Ans

Handle and bristle of a tooth brush should not be made of the same material. This is because handle of brush is strong and hard, while bristles are soft and flexible.

Q.44 ‘Avoid plastics as far as possible’. Comment on this advice.

Ans

Plastics are non-biodegradable, i.e. they are not decomposed by micro-organisms. Once introduced into the environment they may take several years to decompose. Plastics when burnt produce poisonous gases. They are dangerous to animals like cows, which swallow these bags from garbage. These bags choke the respiratory system of the animals or form a lining in their stomach and may cause their death. Hence, we should avoid plastics as far as possible.

Q.45 Match the terms of column A correctly with the phrases given in column B.

A B
(i) Polyester (a) Prepared by using wood pulp
(ii) Teflon (b) Used to make non-stick
(iii) Rayon (c) Prepared by using wood pulp
(iv) Nylon (d) Used for making parachutes and stockings

Ans

A B
(i) Polyester (d) Fabrics do not wrinkle easily
(ii) Teflon (c) Used to make non-stick
(iii) Rayon (a) Prepared by using wood pulp
(iv) Nylon (b) Used for making parachutes and stockings

Q.46 ‘Manufacturing synthetic fibres is actually helping in conservation of forests’. Comment.

Ans

Raw material used for making natural fibres is mainly derived from plants. It requires cutting of lots of trees leading to deforestation. On the other hand raw materials required for synthetic fibres are mainly obtained from petrochemicals. It means for manufacturing of synthetic fibres, cutting of trees is not needed. Hence, it can be said that ‘manufacturing synthetic fibres is actually helping in conservation of forest’.

Q.47 Describe an activity to show that thermoplastic is a poor conductor of electricity.

Ans

We can set two circuits to show that thermoplastic is a poor conductor of electricity. For this we need two bulbs, wires, two batteries, a piece of metal and a pipe made of thermoplastic.

Set up the circuits with metal and plastic pipe separately as shown below.

When you turn the switch on the current flows and the bulb glows in the first circuit (circuit with metal) while, bulb does not glow in the second circuit (circuit with plastic).

This activity confirms that thermoplastic is a poor conductor of electricity.

Q.48 Which of the following can be beaten into sheets?
(a) Zinc
(b) Phosphorous
(c) Sulphur
(d) Oxygen

Ans

Correct option is (a) Zinc.
Explanation: Generally metals can be beaten into sheets. Zinc is a metal while phosphorus, sulphur and oxygen are non-metals.

Q.49 Which of the following statements is correct?
(a) All metals are ductile.
(b) All non-metals are ductile.
(c) Generally, metals are ductile.
(d) Some non-metals are ductile.

Ans

Correct statement is option (c), Generally, metals are ductile.
Explanation: Mercury is a metal but it cannot be drawn into wires because it is liquid at room temperature. Therefore, it is non-ductile.

Q.50 Fill in the blanks.
(a) Phosphorous is a very ___________ non-metal.
(b) Metals are ________ conductors of heat and __________.
(c) Iron is ________reactive than copper.
(d) Metals react with acids to produce ________ gas.

Ans

(a) reactive
(b) good, electricity
(c) more
(d) hydrogen

Q.51 Mark ‘T’ if the statement is true and ‘F’ if it is false.
(a) Generally, non-metals react with acids. ( )
(b) Sodium is a very reactive metal. ( )
(c) Copper displaces zinc from zinc sulphate solution. ( )
(d) Coal can be drawn into wires. ( )

Ans

(a) Generally, non-metals react with acids. (F)
(b) Sodium is a very reactive metal. (T)
(c) Copper displaces zinc from zinc sulphate solution. (F)
(d) Coal can be drawn into wires. (F)

Q.52 Some properties are listed in the following table. Distinguish between metals and non-metals on the basis of these properties.

Properties Metals Non-metals
  1. Appearance
  1. Hardness
  1. Malleability
  1. Ductility
  1. Heat Conduction
  1. Conduction of Electricity

Ans

Properties Metals Non-metals
  1. Appearance
Lustrous Non-lustrous
  1. Hardness
Hard Soft
  1. Malleability
Can be beaten into thin sheets Cannot be beaten into thin sheets
  1. Ductility
Can be drawn into wires Cannot be drawn into wires
  1. Heat Conduction
Good conductors of heat Poor conductors of heat
  1. Conduction of Electricity
Good conductors of electricity Poor conductors of electricity

Q.53 Give reasons for the following.
(a) Aluminium foils are used to wrap food items.
(b) Immersion rods for heating liquids are made up of metallic substances.
(c) Copper cannot displace zinc from its salt solution.
(d) Sodium and potassium are stored in kerosene.

Ans

(a) Aluminium is a highly malleable cheap metal which does not react with food items. Therefore, aluminium foils are used to wrap food items.
(b) Metals are good conductors of heat and electricity. Therefore, immersion rods used for heating liquids are made up of metallic substances.
(c) A less reactive metal cannot replace more reactive metal from its salt solution. Since, copper is less reactive than zinc, therefore, it cannot replace zinc from its salt solution.
(d) Sodium and potassium are very reactive metals. They react vigorously with oxygen and water. A lot of heat is generated in the reaction. If kept in open, they react with oxygen and moisture present in air and catch fire immediately. Therefore, sodium and potassium are stored in kerosene.

Q.54 Can you store lemon pickle in an aluminium utensil? Explain.

Ans

No, lemon pickle cannot be stored in an aluminium utensil. This is because lemon pickle contains acid which reacts with aluminium (metal) to liberate hydrogen gas. The liberated hydrogen gas can spoil lemon pickle.

Q.55 Match the substances given in column A with their uses given in column B.

A B
(i) Gold (a) Thermometers
(ii) Iron (b) Electric wire
(iii) Aluminium (c) Wrapping food
(iv) Carbon (d) Jewellery
(v) Copper (e) Machinery
(vi) Mercury (f) Fuel

Ans

A B
(i) Gold (d) Jewellery
(ii) Iron (e) Machinery
(iii) Aluminium (c) Wrapping food
(iv) Carbon (f) Fuel
(v) Copper (b) Electric wire
(vi) Mercury (a) Thermometers

Q.56 What happens when

(a) Dilute sulphuric acid is poured on a copper plate?

(b) Iron nails are placed in copper sulphate solution?

Write word equations of the reactions involved.

Ans

(a) When sulphuric acid is poured on a copper plate, the copper metal reacts with sulphuric acid to form copper sulphate and hydrogen gas.

(b) Iron is more reactive than copper therefore, it displaces copper from copper sulphate solution to form iron sulphate.

Initially, copper sulpahte solution is blue in colour. when iron nail is placed in it, the blue colour of copper sulphate solution fades. The solution turns green due to formation of iron sulphate. Copper gets deposited on iron nail.

Q.57 Saloni took a piece of burning charcoal and collected the gas evolved in a test tube.

(a) How will she find the nature of the gas?

(b) Write down word equations of all the reactions taking place in this process.

Ans

(a) To find the nature of the gas, add a small amount of water into the test tube in which gas is collected. Quickly cover the test tube and shake it well. Now test the solution with red and blue litmus paper one by one. You will observe that solution turns blue litmus red while red litmus remains unaffected. This confirms that gas evolved during burning of charcoal is acidic in nature.

(b) Name of the product formed in the reaction of charcoal with oxygen is carbon dioxide.

C + O2CO2Carbon Oxygen Carbondioxide(fromcharcoal)

When carbon dioxide is dissolved in water, carbonic acid is formed. The carbonic acid turns blue litmus paper red. Generally, oxides of non-metals are acidic in nature.

CO2+ H2O H2CO3Carbondioxide Water Carbonicacid (Turnsbluelitmusred)

Q.58 One day Reeta went to a jeweller’s shop with her mother. Her mother gave an old gold jewellery to the goldsmith to polish. Next day, when they brought the jewellery back, they found that there was a slight loss in its weight. Can you suggest a reason for the loss in weight?

Ans

To polish gold jewellery, it is dipped in a mixture of acids called aqua regia (a mixture of hydrochloric acid and nitric acid). The outer layer of gold jewellery dissolves in the acid mixture and the inner shiny layer appears. Due to dissolution of upper layer of gold, its weight gets reduced.

Q.59 What are advantages of using CNG and LPG as fuels?

Ans

The advantages of using LPG as a fuel are as follows:

  1. LPG burns easily with smokeless flame. It is a clean fuel.
  2. LPG has high calorific value.
  3. LPG does not produce any poisonous gases on burning.
  4. LPG does not leave behind any solid residue on burning.

The advantages of using CNG as a fuel are as follows:

  1. CNG burns easily with smokeless flame. It is a clean fuel.
  2. CNG can be used directly for burning in homes and factories.
  3. CNG is used as a fuel for vehicles because it is less polluting.
  4. CNG is used as a fuel in thermal power stations for generating electricity.

Q.60 Name the petroleum product used for surfacing of roads.

Ans

Bitumen is used for surfacing of roads.

Q.61 Describe how coal is formed from dead vegetation. What is the process called?

Ans

  1. About 300 million years ago the earth had dense forest in low lying wetlands. Due to natural processes like flooding and earthquakes, forests got buried under the soil. With the passage of time, as more and more soil got deposited on them, the buried plants were compressed. Deep down the earth, pressure and temperature was very high. Under high pressure and temperature, slowly plants got converted into coal.
  2. The slow process of conversion of dead vegetation into coal is called carbonisation.

Q.62 Fill in the blanks.

    1. Fossil fuels are __________, __________ and ___________.
    2. Process of separation of different constituents from petroleum is called _____________.
    3. Least polluting fuel for vehicle is _________.

Ans

  1. coal, petroleum, natural gas
  2. refining
  3. CNG

Q.63 Tick True/False against the following statements.

    1. Fossil fuels can be made in the laboratory. (T/F)
    2. CNG is more polluting fuel than petrol. (T/F)
    3. Coke is almost pure form of carbon. (T/F)
    4. Coal tar is a mixture of various substances.(T/F)
    5. Kerosene is not a fossil fuel. (T/F)

Ans

  1. Fossil fuels can be made in the laboratory. (F)
  2. CNG is more polluting fuel than petrol. (F)
  3. Coke is almost pure form of carbon. (T)
  4. Coal tar is a mixture of various substances. (T)
  5. Kerosene is not a fossil fuel. (F)

Q.64 Explain why fossil fuels are exhaustible natural resources.

Ans

The amount of fossil fuels available in nature is limited. The formation of fossil fuels takes millions of years. Fossil fuels are made by the action of heat and pressure on the remains of dead vegetation and animals. The rate at which we are using them is very fast as compared to their rate of formation. Therefore, fossil fuels are exhaustible natural resources.

Q.65 Describe characteristics and uses of coke.

Ans

Coke is a black porous substance. It is obtained as residue during destructive distillation of coal. The characteristics of coke are as follows:

  1. Coke is an almost pure form of carbon.
  2. It burns almost with no smoke.
  3. It is a non-conductor of heat and electricity.

Uses of coke are as follows:

  1. Coke is used in the extraction of metals.
  2. It is also used in the manufacturing of steel.

Q.66 Explain the process of formation of petroleum.

Ans

  • Petroleum originated from the remains of sea organisms.
  • When sea organisms died, they got buried at the bottom of sea.
  • With passage of time, their bodies got covered with mud.
  • Pressure and temperature rose as they got buried lower and lower.
  • Due to the effect of heat, pressure and action of bacteria in the absence of air, the buried remains decomposed very slowly forming petroleum.

Q.67 The following table shows the total power shortage in India from 1991-1997. Show the data in the form of a graph. Plot shortage percentage for the years on the Y-axis and the year on the X-axis.

S.No. Year Shortage (%)
1 1991 7.9
2 1992 7.8
3 1993 8.3
4 1994 7.4
5 1995 7.1
6 1996 9.2
7 1997 11.5

Ans

Q.68 List conditions under which combustion can take place.

Ans

Necessary conditions for combustion are:

  1. Fuel: Fuel is the raw material for combustion.
  2. Air: Combustion takes place only in the presence of air or oxygen.
  3. Heat: Heat is required to raise the temperature of a fuel so that it attains its ignition temperature. Ignition temperature is the minimum temperature at which a substance catches fire and starts burning.

Q.69 Fill in the blanks.

    1. Burning of wood and coal causes ___________ of air.
    2. A liquid fuel, used in homes is _______.
    3. Fuel must be heated to its __________ __________ before it starts burning.
    4. Fire produced by oil cannot be controlled by ______________.

Ans

1. pollution
2. LPG
3. ignition temperature
4. water

Q.70 Explain how the use of CNG in automobiles has reduced pollution in our cities.

Ans

Combustion of fuels like petrol and diesel produces un-burnt particles along with carbon monoxide gas. These pollutants cause many respiratory problems. Oxides of sulphur and nitrogen are also released due to combustion of petrol and diesel. Oxides of sulphur and nitrogen cause acid rain. CNG produces less harmful products and does not cause much of air pollution. It is therefore, called as cleaner fuel.

Q.71 Compare LPG and wood as fuels.

Ans

Comparison of LPG and wood as fuels

LPG as fuel Wood as Fuel
Burning of LPG does not produce smoke and other pollutants. It is a cleaner fuel. Burning of wood produces a lot of smoke and other pollutants which may cause respiratory problems.
LPG is a liquefied form of petroleum. Wood is obtained from trees. Therefore, use of wood as fuels may lead to deforestation.
Calorific value of LPG is very high (55000 kJ/kg). Calorific value of wood is low (17000 -22000 kJ/kg).

Q.72 Give reasons.

    1. Water is not used to control fires involving electrical equipment.
    2. LPG is a better domestic fuel than wood.
    3. Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

Ans

  1. Water cannot be used to put off fire caused by the electrical equipments because it may conduct electricity and harm those trying to douse the fire.
  2. Burning of wood produces a lot of smoke and un-burnt particles cause respiratory problems. LPG does not produce smoke and its calorific value is much higher as compared to wood. Therefore, LPG is a better domestic fuel than wood.
  3. For burning, a combustible substance must attain its ignition temperature. Piece of paper wrapped around an aluminium pipe does not catch fire on heating because it is unable to reach its ignition temperature due to transfer of heat to aluminium pipe which is a good conductor of heat.

Q.73 Make a labelled diagram of a candle flame.

Ans

Q.74 Name the unit in which the calorific value of a fuel is expressed.

Ans

Calorific value of a fuel is expressed in Kilo Joule per kg (kJ/kg).

Q.75 Explain how CO2 is able to control fires.

Ans

  • Carbon dioxide is the best extinguisher for fires involving electrical equipments and inflammable materials like petrol because CO2 is heavier than oxygen and it covers the fire like a blanket.
  • It controls fire by cutting off the supply of oxygen.

Q.76 It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

Ans

A heap of leaves contains a lot of water. Due to presence of water, ignition temperature of green leaves is not reached easily. Hence, it is difficult to burn a heap of green leaves. Dry leaves do not contain water. Therefore, ignition temperatures of dry leaves are easily attained and catch fire easily.

Q.77 Which zone of a flame does a goldsmith use for melting gold and silver and why?

Ans

Goldsmith uses outermost zone of the flame for melting gold and silver. This is because the outermost zone of the flame is hottest part of the flame. In this zone complete combustion takes place.

Q.78 In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the flame.

Ans

Calorific value of the fuel is the amount of heat energy produced on complete combustion of 1 kg of fuel.

Amount of heat produced by 4.5 kg of fuel = 180,000 kJ

Therefore, heat produced by 1 kg of fuel = 180,000/4.5

= 40,000 kJ/kg

Hence, calorific value of given fuel is 40,000 kJ/kg.

Q.79 Can the process of rusting be called combustion? Discuss.

Ans

Combustion is the chemical process in which a substance reacts with oxygen and gives out energy in the form of either heat or light or both. It is a very fast process. On the other hand, rusting is a slow process which involves the conversion of iron into rust in presence of air (oxygen) and water. In both the processes, oxygen is common reactant.

Q.80 Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the wick in the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of the flame. Whose water will get heated in a shorter time?

Ans

The water in the Ramesh’s beaker will require shorter time for heating. This is because Ramesh kept beaker in the outermost part of the flame which is hottest zone of the candle flame. The water in Abida’s beaker will take comparatively longer to heat up as the yellow zone is less hot when compared to the outermost part of the flame.

Q.81 What are the different ways in which water gets contaminated?

Ans

Water gets contaminated by following ways:

  1. Excessive amounts of pesticides and fertilisers are being used to increase crop production. These chemicals are carried away to the water bodies by rains and floods which contaminate water.
  2. Harmful chemicals discharged from industries pollute water bodies.
  3. Water gets contaminated by the discharge of sewage into rivers.

Q.82 At an individual level, how can you help reduce air pollution?

Ans

An individual can reduce air pollution by:

  1. Using public transport and encouraging car pooling.
  2. Using bicycle for short distances.
  3. Using CNG and unleaded petrol instead of leaded petrol and diesel in automobiles.
  4. Using alternative fuels like solar energy, wind energy and hydropower instead of fossil fuels.
  5. Planting trees and nurturing the ones already present.

Q.83 Clear, transparent water is always fit for drinking. Comment.

Ans

No, clear and transparent water is not always fit for drinking. Water which looks clean may still have disease carrying microorganisms and dissolved impurities. Therefore, it is essential to purify water before drinking.

Q.84 You are a member of the municipal body of your town.

Make a list of measures that would help your town to ensure the supply of clean water to all its residents.

Ans

As a member of the municipal body of my town, I would take certain necessary actions to ensure the supply of clean water.

  1. Water must be treated properly to make it fit for use.
  2. Chlorination must be done to disinfect the drinking water supplies.
  3. Rain water harvesting will be made mandatory in every house.

Q.85 Explain the differences between pure air and polluted air.

Ans

Pure air Polluted air
Pure air contains 78% nitrogen, 21% oxygen and 0.03% carbon dioxide. Water vapour and other gases are present in traces. Composition of air is altered by the presence of harmful substances such as oxides of nitrogen, sulphur dioxide, carbon monoxide, particulate matters, etc.
It does not have any harmful substances. Increased level of harmful substances and poisonous gases are present.
Pure air is fit for breathing and does not cause any respiratory problems. Polluted air is unfit for breathing and causes many respiratory problems like asthma.

Q.86 Explain circumstances leading to acid rain. How does acid rain affect us?

Ans

Sulphur dioxide and oxides of nitrogen are produced from various industries. These gases react with water vapour present in the air to form sulphuric acid and nitric acid, respectively. These acids come down with the rain water and this process is called acid rain.

Acid rain destroys plants and vegetation. The water of the rivers and the lakes becomes acidic and this causes the destruction of the aquatic life. Acid rain also destroys the ancient buildings, monuments and sculptures due to the continuous erosion of the marble.

Q.87 Which of the following is not a greenhouse gas?

    1. Carbon dioxide
    2. Sulphur dioxide
    3. Methane
    4. Nitrogen

Ans

The correct options are (d).

Q.88 Describe the ‘Green House Effect’ in your own words.

Ans

Gases like carbon dioxide, methane, water vapour etc. present in the atmosphere are called green house gases. These gases trap the heat from Sun and prevent it from escaping into the outer space. It helps in keeping earth warm and is essential for life on earth. But increase in the concentration of these gases leads to excessive increase in Earth’s temperature leading to global warming.

Q.89 Prepare a brief speech on global warming. You have to deliver the speech in your class.

Ans

Global warming is an increase in the average temperature of Earth’s atmosphere. It is caused due to increased levels of green house gases like carbon dioxide, oxides of nitrogen, water vapour, methane, etc. These gases absorb solar radiation and prevent it from escaping into the outer space thereby, increasing Earth’s average temperature.

This continuous warming has lead to an increase in the temperature of Earth’s surface by 0.6°C approx. in the last three decades. Due to a rise in the temperature, the polar ice is melting and is threatening the survival of the polar bears.

The melting of the polar ice causes a rise in the sea level. It harms the coastal life and vegetation.

Q.90 Describe the threat to the beauty of the Taj Mahal.

Ans

Sulphur dioxide and nitrogen dioxide produced from the industries like rubber processing industry, Mathura oil refinery etc. located in and around Agra are responsible for the discolouring of white marble of Taj Mahal. Sulphur dioxide and nitrogen dioxide gases released from the industries, react with water vapour present in the air to form sulphuric acid and nitric acid, respectively. The acids drop down with rain, making rain acidic. Acid rain corrodes the marble of the Taj Mahal. The phenomenon is also known as ‘Marble Cancer’. Suspended particulate matters emitted by ‘Mathura oil refinery’ have also contributed towards the yellowing of the marble.

Q.90 Why does the increased level of nutrients in the water affect the survival of aquatic organism?

Ans

When water bodies are nutrient enriched, then a dense growth of algae occurs. When these algae die, the decomposers decompose them by using a lot of oxygen. It results in decrease in concentration of oxygen in water resulting in death of aquatic organisms.

Q.91 Fill in the blanks.
(a) Microorganisms can be seen with the help of a __________.
(b) Blue green algae fix _________ directly from air and enhance fertility of soil.
(c) Alcohol is produced with the help of ________.
(d) Cholera is caused by __________.

Ans

(a) microscope
(b) nitrogen
(c) yeast
(d) bacteria

Q.92 Tick the correct answer.
a) Yeast is used in the production of
(i) sugar
(ii) alcohol
(iii) hydrochloric acid
(iv) oxygen

(b) The following is an antibiotic
(i) Sodium bicarbonate
(ii) Streptomycin
(iii) Alcohol
(iv) Yeast

(c) Carrier of malaria-causing protozoan is
(i) female Anopheles mosquito
(ii) cockroach
(iii) housefly
(iv) butterfly

(d) The most common carrier of communicable diseases is
(i) ant
(ii) housefly
(iii) dragonfly
(iv) spider

(e) The bread or idli dough rises because of
(i) heat
(ii) grinding
(iii) growth of yeast cells
(iv) kneading

(f) The process of conversion of sugar into alcohol is called
(i) nitrogen fixation
(ii) moulding
(iii) fermentation
(iv) infection

Ans

(a) (ii) alcohol
(b) (ii) Streptomycin
(c) (i) female Anopheles mosquito
(d) (ii) housefly
(e) (iii) growth of yeast cells
(f) (iii) fermentation

Q.93 Match the organisms in Column A with their action in Column B.

A

B

(i) Bacteria (a) Fixing nitrogen
(ii) Rhizobium (b) Setting of curd
(iii) Lactobacillus (c) Baking of bread
(iv) Yeast (d) Causing malaria
(v) A protozoan (e) Causing cholera
(vi) A virus (f) Causing AIDS
(g) Producing antibodies

Ans

A

B

(i) Bacteria (e) Causing cholera
(ii) Rhizobium (a) Fixing nitrogen
(iii) Lactobacillus (b) Setting of curd
(iv) Yeast (c) Baking of bread
(v) A protozoan (d) Causing malaria
(vi) A virus (f) Causing AIDS

Q.94 Can microorganisms be seen with the naked eye? If not, how can they be seen?

Ans

No, microorganisms cannot be seen with the naked eye. They are too small to be seen with naked eye. They can be seen through magnifying lens and microscope. Microorganisms, such as protozoans, present in water can be seen under a microscope. Fungus, that grows on bread, can be seen with the help of a magnifying lens.

Q.95 What are the major groups of microorganisms?

Ans

Microorganisms are classified into four major groups. The four major groups of microorganisms are:

  1. Bacteria
  2. Fungi
  3. Protozoa
  4. Algae

Q.96 Name the microorganisms which can fix atmospheric nitrogen in the soil.

Ans

The microorganisms that fix atmospheric nitrogen in the soil are Rhizobium and blue-green algae. These microorganisms are commonly called biological nitrogen fixers that fix atmospheric nitrogen to enrich soil with nitrogen and increase the fertility of soil.

Q.97 Write 10 lines on the usefulness of microorganisms in our lives.

Ans

Following are some functions of microorganisms that are important to us:

  1. At home, microorganism like yeast is used in the preparation of bread, cake, idli, dosa, etc.
  2. In baking industry, microorganism like yeast is used in making breads, pastries and cakes.
  3. A bacterium, Lactobacillus, helps in the setting of curd from milk.
  4. Microorganism like yeast is used in the production of alcohols and wine on large scale.
  5. In agriculture, microorganisms like Rhizobium and blue green algae help in the fixation of atmospheric nitrogen to enrich the fertility of soil.
  6. In medicine, microorganisms such as bacteria and fungi play an important role in the preparation of antibiotics.
  7. Microorganisms, such as certain bacteria, help in cleaning the environment by decomposing organic wastes into harmless and usable substances.
  8. They are also used in the preparation of vaccines to protect humans and other animals from various diseases.
  9. Certain microorganisms play an important role in the sewage treatment plant.
  10. Antibiotics prepared from microorganisms are also used to prevent microbial infection in the animals of livestock and poultry and to control various diseases in plants.

Q.98 Write a short paragraph on the harmful effects of microorganisms.

Ans

Microorganisms cause harm in many ways. Some of the harmful effects of microorganisms include causing diseases in humans, animals and plants and spoiling of food, clothes and leather. The disease causing microorganisms are called pathogens. Some bacterial microorganisms cause tuberculosis, typhoid, cholera, etc. in human beings. Diseases, like measles, chickenpox, polio, hepatitis, etc., are caused by viruses in human beings. In cattle, anthrax is caused by bacteria and foot and mouth disease is caused by fungi. In plants, disease like citrus canker, rust of wheat and yellow vein mosaic of bhindi are caused by bacteria, fungi and virus, respectively and reduces the yield of crops. Some microorganisms, like bacteria and fungi, spoil food by producing toxic substances. When these microorganisms are ingested by human beings along with the spoiled food, food poisoning is caused.

Q.99 What are antibiotics? What precautions must be taken while taking antibiotics?

Ans

Antibiotics are the medicines that are produced by the microorganisms to kill the disease causing microorganisms. The microorganisms that are used to prepare the antibiotics are bacteria and fungi.

The precautions that are to be taken while taking antibiotics are:

  1. It should be taken under a medical supervision.
  2. A complete course of antibiotics should be taken as prescribed by the physician.
  3. A right amount of antibiotics should be taken at right time as prescribed.
  4. Intake of wrong dose would make the antibiotic less effective.
  5. If not required, antibiotics should not be consumed as it may make body resistant, when it is needed in future and kill the beneficial bacteria present in the body.

Q.100 Indicate whether the following statements are True (T) or False (F).
(a) Unicellular organisms have one-celled body. (T/F)
(b) Muscle cells are branched. (T/F)
(c) The basic living unit of an organism is an organ. (T/F)
(d) Amoeba has irregular shape. (T/F)

Ans

(a) Unicellular organisms have one-celled body. (T)

(b) Muscle cells are branched. (F)

(c) The basic living unit of an organism is an organ. (F)

(d) Amoeba has irregular shape. (T)

Q.101 Make a sketch of human nerve cell. What function do nerve cells perform?

Ans

The nerve cell receives and transfers messages to control and coordinate the working of different parts of the body.

Q.102 Write short notes on the following.
(a) Cytoplasm
(b) Nucleus of a cell

Ans

(a) Cytoplasm

In a cell, a jelly-like substance present between the nucleus and the cell membrane is called cytoplasm. It contains various cell organelles, such as mitochondria, golgi apparatus, endoplasmic reticulum, lysosomes, etc.

(b) Nucleus of a cell

Nucleus is generally spherical in shape and located centrally in a cell. A porous membrane, called nuclear membrane, separates the nucleus from the cytoplasm. Nuclear membrane allows the exchange of materials between the nucleus and the cytoplasm.

Nucleus contains a smaller spherical body called nucleolus and thread-like structures called chromosomes. Chromosomes carry genes and help in the transfer of characters from one generation to the next generation. In addition to the function of inheritance of characters, nucleus regulates all the activities of a cell.

Q.103 Which part of the cell contains organelles?

Ans

Cytoplasm of the cell contains many organelles.

Q.104 Make sketches of animal and plant cells. State three differences between them.

Ans

Animal cell

Plant Cell

Differences between animal cell and pant cell are as follows:

Parts of cell Animal Cell Plant Cell
Cell wall Absent Present
Plastids Absent Present
Vacuoles Many but small in size One big central

Q.105 State the difference between eukaryotes and prokaryotes.

Ans

Following are the differences between eukaryotes and prokaryotes:

Features Eukaryotes Prokaryotes
Number of cells Unicellular or Multicellular Unicellular
Nucleus Well defined, and true nucleus Poorly defined, and a primitive nucleus
Nuclear membrane Present Absent

Q.106 Where are chromosomes found in a cell? State their function.

Ans

Chromosomes are found in the nucleus of a cell. Chromosomes are thread-like structures that carry genes which play a vital role in the inheritance of characters from parents to their offspring.

Q.107 ‘Cells are the basic structural units of living organisms.’ Explain.

Ans

In a living organism, various organs are present which perform various functions such as digestion, respiration, reproduction, movement, etc. Organs are specific for their function that they perform. For example, leaves carry out photosynthesis, ovary produces ova, etc. In an organism, specific tissues are present that facilitate it to perform its specific function. Furthermore, a tissue is composed of similar types of cells. Hence, we can say that cells are the basic structural unit of living organisms.

Q.108 Explain why chloroplasts are found only in plant cells?

Ans

The chloroplast is a type of plastid that contains green coloured pigment, chlorophyll. Chlorophyll is one of the essential requirements for photosynthesis to occur. Plastids are the organelles that are present only in plant cells. Hence, chloroplasts are found only in plant cells.

Q.109 Complete the crossword with the help of clues given below.
Across
1. This is necessary for photosynthesis.
3. Term for component present in the cytoplasm.
6. The living substance in the cell.
8. Units of inheritance present on the chromosome.

Down
1. Green plastids.
2. Formed by collection of tissues.
4. It separates the contents of the cell from the surrounding medium.
5. Empty structure in the cytoplasm.
7. A group of cells.

Ans

Q.110 Give two examples each of situations in which you push or pull to change the state of motion of objects.

Ans

Examples of Push:

  1. A suitcase at rest is pushed to move from one place to another. This changes the state of motion of the suitcase.
  2. A football is pushed by a player using his foot.This changes the state of motion of the football.

Examples of Pull:

  1. To open a drawer, it is pulled out. This changes the state of motion of the drawer.
  2. Rope is pulled to draw water from a well. This changes the state of motion of the water bucket.

Q.111 Give two examples of situations in which applied force causes a change in the shape of an object.

Ans

  1. Squeezing of tin container.
  2. Pressing clay by hands.

Q.112 Fill in the blanks in the following statements.

(a) To draw water from a well we have to ___ on the rope.
(b) A charged body ___ an uncharged body towards it.
(c) To move a loaded trolley we have to ___ it.
(d) The north pole of a magnet ___ the north pole of another magnet.

Ans

(a) To draw water from a well we have to pull on the rope.
(b) A charged body attracts an uncharged body towards it.
(c) To move a loaded trolley we have to either push or pull it.
(d) The north pole of a magnet repels the north pole of another magnet.

Q.113 An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on this information fill up the gaps in the following statements using the following terms.

muscular, contact, non-contact, gravity, friction, shape, attraction

(a) To stretch the bow, the archer applies a force that causes a change in its __________.
(b) The force applied by the archer to stretch the bow is an example of __________ force.
(c) The type of force responsible for a change in the state of motion of the arrow is an example of a __________ force.
(d) While the arrow moves towards its target, the forces acting on it are due to __________ and that due to __________ of air.

Ans

(a) To stretch the bow, the archer applies a force that causes a change in its shape.
(b) The force applied by the archer to stretch the bow is an example of muscular force.
(c) The type of force responsible for a change in the state of motion of the arrow is an example of a contact force.
(d) While the arrow moves towards its target, the forces acting on it are due to gravity and that due to friction of air.

Q.114 In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.

(a) Squeezing a piece of lemon between the fingers to extract its juice.
(b) Taking out paste from a toothpaste tube.
(c) A load suspended from a spring while its other end is on a hook fixed to a wall.
(d) An athlete making a high jump to clear the bar at a certain height.

Ans

(a) Muscular force is used in squeezing a piece of lemon. This muscular force acts on the lemon. Thus, the shape of the lemon gets changed.
(b) To take out paste from a toothpaste tube, muscular force is used and it acts on the toothpaste tube. Thus, the shape of the tube gets changed.
(c) The suspended load exerts a force on the spring and pushes the spring in downward direction. Thus, the spring gets stretched and its shape gets changed.
(d) In a high jump, an athlete pushes the ground with his feet and exerts a muscular force on the ground. As a result, he can jump over the bar. Thus, his state of motion gets changed.

Q.115 A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?

Ans

A blacksmith uses muscular force when he hammers on a hot piece of iron. Due to this muscular force, the shape of the iron gets changed and iron piece can be given a desired shape.

Q.116 An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the balloon and the wall?

Ans

An inflated balloon gets charged when it is rubbed with a piece of synthetic cloth. A charged body attracts an uncharged body. When this balloon is pressed against a wall, it sticks to the wall. The force acting between the charged balloon and the wall is the electrostatic force due to which it was stuck.

Q.117 Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.

Ans

When we hold a plastic bucket filled with water above the ground, there are two forces act on it. First is the gravitational force which acts downwards and second is the muscular force exerted by us which acts in the upward direction. The two forces are equal in magnitude but opposite in direction. Hence, the net force on the bucket is zero. Therefore, there is no change in its state of motion.

Q.118 A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.

Ans

  1. The force of gravity which tries to pull the rocket downwards.
  2. The force of friction due to earth’s atmosphere, which opposes the motion of the rocket.

Q.119 When we press the bulb of a dropper with its nozzle kept in water, air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to

(a) pressure of water.
(b) gravity of the earth.
(c) shape of rubber bulb.
(d) atmospheric pressure.

Ans

The correct option is (d).

Explanation: When the air filled in the nozzle is removed, the atmospheric pressure, which is acting on the water, forces the water to fill the nozzle of the dropper.

Q.120 Fill in the blanks.

(a) Friction opposes the _____________ between the surfaces in contact with each other.
(b) Friction depends on the _____________ of surfaces.
(c) Friction produces __________.
(d) Sprinkling of powder on the carrom board ________ friction.
(e) Sliding friction is ___________ than the static friction.

Ans

(a) Friction opposes the motion between the surfaces in contact with each other.
(b) Friction depends on the nature of surfaces.
(c) Friction produces heat.
(d) Sprinkling of powder on the carrom board reduces friction.
(e) Sliding friction is less than the static friction.

Q.121 Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order. Their arrangements are given below. Choose the correct arrangement.

(a) rolling, static, sliding
(b) rolling, sliding, static
(c) static, sliding, rolling
(d) sliding, static, rolling

Ans

The correct option is (c).

Explanation: The force of friction acts due to irregularities between two surfaces. Static friction acts when an object is moved from rest. Sliding friction comes into play when an object slides over the surface of another object. In sliding friction, the time given for interlocking is very less. Hence, interlocking is not strong. Therefore, less force is required to overcome this interlocking. Due to this, sliding friction is less than static friction. Also, the area of contact in case of rolling friction is smallest as compared to static or sliding friction. This area of contact changes gradually because of rolling. Thus, rolling friction is lesser than both static and sliding friction. So, the correct sequence is – static, sliding, rolling.

Q.122 Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be

(a) wet marble floor, dry marble floor, newspaper and towel.
(b) newspaper, towel, dry marble floor, wet marble floor.
(c) towel, newspaper, dry marble floor, wet marble floor.
(d) wet marble floor, dry marble floor, towel, newspaper

Ans

The correct option is (a).

Explanation: Frictional force depends on the nature of surfaces in contact. The rougher the surface, the more is the friction between the surfaces in contact. According to roughness present in the given surfaces, we can arrange the force of friction acting between different surfaces as, wet marble floor, dry marble floor, newspaper and towel.

Q.123 Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.

Ans

When the book slides on the desk, the force of friction acts between the book and the surface of the desk. The direction of frictional force is opposite to the direction of the motion of book and acts in upward direction.

Q.124 You spill a bucket of soapy water on a marble floor accidently. Would it make it easier or more difficult for you to walk on the floor? Why?

Ans

A person can walk because of the friction present between our feet and the ground. In order to walk, the person pushes the ground in the backward direction with his feet and the force of friction pushes it in the forward direction. When there is soapy water spilled on the floor, the force of friction between the ground and feet decreases. Thus, it becomes difficult to walk on the soapy floor.

Q.125 Explain why sportsmen use shoes with spikes.

Ans

A sportsman wears shoes with spikes because these shoes give him a better grip while running. Due to this, force of friction between the shoes and the ground increases.

Q.126 Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?

Ans

When a heavy object is kept on the floor, the interlocking of irregularities on the surfaces of box and floor become very powerful. It is because the two surfaces in contact are pressed harder. Thus, more force is required to overcome the interlocking. Therefore, to push the heavier box, Seema has to apply a larger force than Iqbal.

Q.127 Explain why sliding friction is less than static friction.

Ans

Friction arises between two objects when irregularities present in the surfaces get interlocked with each other. In the case of sliding, the time given for interlocking is very small. Therefore, interlocking is not strong. Hence, less force is required to overcome from this interlocking. Thus, sliding friction is less than static friction.

Q.128 Give examples to show that friction is both a friend and a foe.

Ans

Friction as a Friend:

  1. Due to friction, a person is able to walk.
  2. Friction between the tip of a pen and a paper helps us to write.

Friction as a Foe:

  1. Due to friction, tyres and soles of shoes wear out.
  2. Friction between different parts of machines can generate heat. This can damage the parts of machines.

Q.129 Explain why objects moving in fluids must have special shapes.

Ans

An object moving through a fluid experiences an opposing force which tries to oppose its motion. This opposing force is also called the drag force. This force depends on the shape of the body. By giving the body a special shape, this force can be minimised. Thus, it becomes easier for the body to move through the fluid.

Q.130 Choose the correct answer.

Sound can travel through
(a) gases only
(b) solids only
(c) liquids only
(d) solids, liquids and gases

Ans

The correct option is (d).

Explanation: Sound cannot travel through vacuum. Solids, liquids and gases provide the medium for sound to travel.

Q.131 Voice of which of the following is likely to have minimum frequency?

(a) Baby girl
(b) Baby boy
(c) A man
(d) A woman

Ans

The correct option is (c).

Explanation: The pitch of an adult man is lower as compared to baby boy, baby girl and woman. Also, frequency of a sound is directly proportional to its pitch. Hence, man’s voice is of minimum frequency in this case.

Q.132 In the following statements, tick ‘T’ against those which are true, and ‘F’ against those which are false.

(a) Sound cannot travel in vacuum. (T/F)
(b) The number of oscillations per second of a vibrating object is called its time period. (T/F)
(c) If the amplitude of vibration is large, sound is feeble. (T/F)
(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)
(e) The lower the frequency of vibration, the higher is the pitch. (T/F)
(f) Unwanted or unpleasant sound is termed as music. (T/F)
(g) Noise pollution may cause partial hearing impairment. (T/F)

Ans

(a) Sound cannot travel in vacuum. (T)
Explanation: Sound travels in the presence of a medium.
(b) The number of oscillations per second of a vibrating object is called its time period. (F)
Explanation: The number of oscillations per second of a vibrating object is known as its frequency.
(c) If the amplitude of vibration is large, sound is feeble. (F)
Explanation: Loudness of a sound is directly proportional to the square of the amplitude of its vibration. When the amplitude of vibration of a sound is large, the sound is called very loud. The sound is feeble for small amplitude.
(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T)
Explanation: A human can hear a sound whose frequency comes in the range of 20 Hz − 20,000 Hz.
(e) The lower the frequency of vibration, the higher is the pitch. (F)
Explanation: The pitch of a sound is proportional to its frequency.
(f) Unwanted or unpleasant sound is termed as music. (F)
Explanation: Unwanted or unpleasant sounds are known as noise.
(g) Noise pollution may cause partial hearing impairment. (T)
Explanation: When a loud and unpleasant sound is produced continuously for a long time, then it may affect temporarily our hearing impairment.

Q.133 Fill in the blanks with suitable words.

(a) Time taken by an object to complete one oscillation is called __________.
(b) Loudness is determined by the __________ of vibration.
(c) The unit of frequency is __________
(d) Unwanted sound is called __________ .
(e) Shrillness of a sound is determined by the __________ of vibration.

Ans

(a) Time taken by an object to complete one oscillation is called time period.
(b) Loudness is determined by the amplitude of vibration.
(c) The unit of frequency is hertz (Hz).
(d) Unwanted sound is called noise.
(e) Shrillness of a sound is determined by the frequency of vibration.

Q.134 A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

Ans

Number of oscillations = 40 Total time taken = 4 s The number of oscillations per second is called frequency. Frequency = Numberofoscillations Totaltimetaken = 40 4s = 10Hz Timeperiod= 1 Frequency = 1 10Hz = 0.1s MathType@MTEF@5@5@+=feaaguart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0le9yqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeaacaGaaiaabeqaamaabaabaaGceaqabeaacaqGobGaaeyDaiaab2gacaqGIbGaaeyzaiaabkhacaqGGaGaae4BaiaabAgacaqGGaGaae4BaiaabohacaqGJbGaaeyAaiaabYgacaqGSbGaaeyyaiaabshacaqGPbGaae4Baiaab6gacaqGZbGaaeiiaiaab2dacaqGGaGaaeinaiaabcdaaeaacaqGubGaae4BaiaabshacaqGHbGaaeiBaiaabccacaqG0bGaaeyAaiaab2gacaqGLbGaaeiiaiaabshacaqGHbGaae4AaiaabwgacaqGUbGaaeiiaiaab2dacaqGGaGaaeinaiaabccacaqGZbaabaGaaeivaiaabIgacaqGLbGaaeiiaiaab6gacaqG1bGaaeyBaiaabkgacaqGLbGaaeOCaiaabccacaqGVbGaaeOzaiaabccacaqGVbGaae4CaiaabogacaqGPbGaaeiBaiaabYgacaqGHbGaaeiDaiaabMgacaqGVbGaaeOBaiaabohacaqGGaGaaeiCaiaabwgacaqGYbGaaeiiaiaabohacaqGLbGaae4yaiaab+gacaqGUbGaaeizaiaabccacaqGPbGaae4CaiaabccacaqGJbGaaeyyaiaabYgacaqGSbGaaeyzaiaabsgacaqGGaGaaeOzaiaabkhacaqGLbGaaeyCaiaabwhacaqGLbGaaeOBaiaabogacaqG5bGaaeOlaiaabccaaeaacqGH0icxcaaMe8UaaeOraiaabkhacaqGLbGaaeyCaiaabwhacaqGLbGaaeOBaiaabogacaqG5bGaaeiiaiaabccacaqG9aWaaSaaaeaacaqGobGaaeyDaiaab2gacaqGIbGaaeyzaiaabkhacaaMe8Uaae4BaiaabAgacaaMe8Uaae4BaiaabohacaqGJbGaaeyAaiaabYgacaqGSbGaaeyyaiaabshacaqGPbGaae4Baiaab6gacaqGZbaabaGaaeivaiaab+gacaqG0bGaaeyyaiaabYgacaaMe8UaaeiDaiaabMgacaqGTbGaaeyzaiaaysW7caqG0bGaaeyyaiaabUgacaqGLbGaaeOBaaaacaqG9aWaaSaaaeaacaqG0aGaaeimaaqaaiaabsdacaaMe8Uaae4CaaaacaqG9aGaaeymaiaabcdacaaMe8UaaeisaiaabQhaaeaacqGHshI3caaMe8UaaeivaiaabMgacaqGTbGaaeyzaiaaysW7caqGWbGaaeyzaiaabkhacaqGPbGaae4BaiaabsgacaaMe8UaaeypaiaaysW7daWcaaqaaiaabgdaaeaacaqGgbGaaeOCaiaabwgacaqGXbGaaeyDaiaabwgacaqGUbGaae4yaiaabMhaaaGaaeypamaalaaabaGaaeymaaqaaiaabgdacaqGWaGaaGjbVlaabIeacaqG6baaaiaab2dacaqGWaGaaeOlaiaabgdacaaMe8Uaae4Caaaaaa@F8AD@

Q.135 The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?

Ans

Frequency of sound produced by mosquito = 500 vibrations per second = 500 Hz Time period = 1 Frequency On putting the given value, Time period = 1 Frequency = 1 500 Hz =0.002 s

Q.136 Identify the part which vibrates to produce sound in the following instruments.
(a) Dholak
(b) Sitar
(c) Flute

Ans

(a) Dholak: It is a musical instrument which consists of a stretched membrane known as its head. When the head is beaten softly, the stretched membrane sets into vibration. Hence, the dholak produces a sound when it is beaten.
(b) Sitar: It is also a musical instrument which consists of stretched strings. When a string is plucked, it sets into vibration. Hence, the sitar produces sound when it is beaten.
(c) Flute: It is a hollow pipe. When air is blown over its mouth, the air inside the pipe is set into vibration. Thus, a pleasant sound is produced.

Q.137 What is the difference between noise and music? Can music become noise sometimes?

Ans

Music: The sound which seems pleasant to ear is called music. For example, the sound produced by violins, pianos, flutes, etc.

Noise: The sound which seems unpleasant to ear is called noise. For example, sound produced by horns of buses and trucks, sound of electrical generators, sound of a gunshot, etc.

Yes. When music is played at very high volume, it becomes noisy.

Q.138 List sources of noise pollution in your surroundings.

Ans

The sources of some noise pollution are as follows:

  1. Horns of vehicles
  2. Loudspeakers
  3. Crackers.

Q.139 Explain in what way noise pollution is harmful to human.

Ans

Noise pollution can cause stress, hypertension, insomnia, hearing loss, etc.

Q.140 Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.

Ans

There will be more noise around the house which is located along the roadside. The effect of noise decreases with the distance between the source and the listener. Thus, it is better to choose the house that is three lanes away from the roadside.

Q.141 Sketch larynx and explain its function in your own words.

Ans

In human beings, sound is produced by the voice box or larynx ,which is at the upper end of the windpipe. Two vocal cords are stretched across the larynx. These vocal cords leave a narrow slit between them for the passage of air. The air from the lungs passes through the slit due to which vocal cords starts vibrating. As we know that vibration of an object is the necessary condition for the production of sound, so the sound is produced due to vibrations of the vocal cords.

Muscles attached to the vocal cords can make the cords tight or loose and can change the type or quality of the voice produced.

Q.142 Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain why?

Ans

The speed of light is more than the speed of sound. Therefore, light reaches us before thundering.

Q.143 Which of the following cannot be charged easily by friction?

(a) A plastic scale
(b) A copper rod
(c) An inflated balloon
(d) A woollen cloth

Ans

The correct option is (b).

Explanation: Charging by friction is possible only in non-conducting materials. As the copper rod is a conducting material therefore, it cannot be charged by friction.

Q.144 When a glass rod is rubbed with a piece of silk cloth the rod

(a) and the cloth both acquire positive charge.
(b) becomes positively charged while the cloth has a negative charge.
(c) and the cloth both acquire negative charge.
(d) becomes negatively charged while the cloth has a positive charge.

Ans

The correct option is (b).”

Explanation: When an object is rubbed with another object, the two objects acquire charges. By convention, it is considered that the charge acquired by the glass rod is positive and charge acquired by the cloth is negative.

Q.145 Write T against true and F against false in the following statements.

(a) Like charges attract each other. (T/F)
(b) A charged glass rod attracts a charged plastic straw. (T/F)
(c) Lightning conductor cannot protect a building from lightning. (T/F)
(d) Earthquakes can be predicted in advance. (T/F)

Ans

(a) False
Explanation: Like charges always repel each other while unlike charges attract each other.

(b) True
Explanation: A charged glass rod has positive charges on its surface while a charged plastic straw has negative charges on its surface.

(c) False
Explanation: The lightning conductor transfers all the atmospheric charge to the Earth. Hence, lightning does not affect the building.

(d) False
Explanation: There is no instrument in the world which can predict the earthquake in advance.

Q.146 Sometimes, a crackling sound is heard while taking off a sweater during winters. Explain.

Ans

The sweater gets charged when it is taken off. It is because of the friction between the sweater and the body. Thus, crackling sound is produced during this process.

Q.147 Explain why a charged body loses its charge if we touch it with our hand.

Ans

When a person touches a charged object, its charge is conducted by his body to the earth. Hence, the charged body loses its charge.

Q.148 Name the scale on which the destructive energy of an earthquake is measured. An earthquake measures 3 on this scale. Would it be recorded by a seismograph? Is it likely to cause much damage?

Ans

Richter scale is used to measure the destructive energy of an earthquake. This scale has the readings from 1 to 10. Seismograph is used to measure the reading of magnitude 3 on the Richter scale. When Richter scale shows a reading of magnitude 3, the earthquake will not cause much damage. An earthquake of magnitudes higher than 5 is considered destructive in nature.

Q.149 Suggest three measures to protect ourselves from lightning.

Ans

  1. Keep away from electrical wires, telephone cables, metal pipes, etc.
  2. If you are driving car, then remain there until the lightning is over. Close the windows of the car immediately.
  3. During lighting, do not take bath in running water.

Q.150 Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon?

Ans

Two charged balloons have similar nature of charges. As like charges always repel each other hence, two charged balloons will repel each other. When a charged balloon is brought near an uncharged balloon, the uncharged balloon acquires charges on its surface due to induction. Since unlike charges attract each other, a charged body always attracts an uncharged body. Therefore, an uncharged balloon is attracted by another charged balloon.

Q.151 Describe with the help of a diagram an instrument which can be used to detect a charged body.

Ans

An electroscope is used to detect a charged body. A simple electroscope is shown in the following figure.

It consists of a metal rod. At one end of the rod, two leaves of aluminium foil are fixed and at the other end, there is a metal disc. The leaves of aluminium foil are placed inside a conical flask and the flask is corked to isolate the leaves from air. When the metal disc is touched with a charged body, the aluminium strips move away from each other. This happens because some of the charges of the body are transferred to the strips through the metal rod. This method of charging a body is called charging by conduction. The nature of charges on both the leaves and the charged body are the similar. Hence, both the leaves of the aluminium foil will move away from each other. If the body was not charged, then the leaves of the foil would remain as they were before. They would not repel each other.

Q.152 List three states in India where earthquakes are more likely to strike.

Ans

Jammu and Kashmir, Gujarat and Assam are three major states of India where earthquakes are more likely to strike.

Q.153 Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself?

Ans

  1. If you are travelling in a bus or a car, then do not come out when an earthquake strikes.
  2. Rush to find an open field away from tall buildings, installations, tall trees, and electric wires and poles during an earthquake.

Q.154 The weather department has predicted that a thunderstorm is likely to occur on a certain day. Suppose you have to go out on that day. Would you carry an umbrella? Explain.

Ans

No, One should not carry an umbrella during thunderstorm. The charge flows from the clouds during thunderstorm,and it can travel through the metallic rod of the umbrella. This may give an electric shock to the person who is carrying it. Thus, it is not safe to carry an umbrella during lightning.

Q.155 Which of the following is NOT a member of the solar system?

(a) An asteroid
(b) A satellite
(c) A constellation
(d) A comet

Ans

The correct option is (c).

Explanation: A constellation is the group of stars which forms recognisable shapes. Hence, it is not a member of the solar system.

Q.156 Which of the following is NOT a planet of the sun?

(a) Sirius
(b) Mercury
(c) Saturn
(d) Earth

Ans

The correct option is (a).

Explanation: Sirius is a star and not a planet of the Sun.

Q.157 Phases of the moon occur because

(a) we can see only that part of the moon which reflects light towards us.
(b) our distance from the moon keeps changing.
(c) the shadow of the Earth covers only a part of the moon’s surface.
(d) the thickness of the moon’s atmosphere is not constant.

Ans

The correct option is (a).

Explanation: Phases of the moon are seen because we can see only that part of the moon which reflects light towards us.

Q.158 Fill in the blanks.

(a) The planet which is farthest from the Sun is ____________ .
(b) The planet which appears reddish in colour is ____________ .
(c) A group of stars that appear to form a pattern in the sky is known as a ____________ .
(d) A celestial body that revolves around a planet is known as __________.
(e) Shooting stars are actually not ____________.
(f) Asteroids are found between the orbits of _________ and _________ .

Ans

(a) The planet which is farthest from the Sun is Neptune.
(b) The planet which appears reddish in colour is Mars.
(c) A group of stars that appear to form a pattern in the sky is known as a constellation.
(d) A celestial body that revolves around a planet is known as a satellite.
(e) Shooting stars are actually not stars.
(f) Asteroids are found between the orbits of Mars and Jupiter.

Q.159 Mark the following statements as true (T) or false (F).

(a) Pole star is a member of the solar system. ( )
(b) Mercury is the smallest planet of the solar system. ( )
(c) Uranus is the farthest planet in the solar system. ( )
(d) INSAT is an artificial satellite. ( )
(e) There are nine planets in the solar system. ( )
(f) Constellation Orion can be seen only with a telescope. ( )

Ans

(a) Pole star is a member of the solar system. (False)
Explanation: Stars are not the member of the solar system. Only Sun and the celestial bodies which are revolving around the Sun are the parts of the solar system.

(b) Mercury is the smallest planet of the solar system. (True)

(c) Uranus is the farthest planet in the solar system. (False)
Explanation: Neptune is the farthest planet in the solar system.

(d) INSAT is an artificial satellite. (True)

(e) There are nine planets in the solar system. (False)
Explanation: There are eight planets in the solar system.

(f) Constellation Orion can be seen only with a telescope. (False)
Explanation: Orion can be seen during winters around late evenings. It is one of the most magnificent constellations in the sky and can be seen by naked eyes.

Q.160 Match items in column A with one or more items in column B.

A

B

(i)

Inner Planets

(a)

Saturn

(ii)

Outer Planets

(b)

Pole Star

(iii)

Constellation

(c)

Great Bear

(iv)

Satellite of the Earth

(d)

Moon

(e)

Earth

(f)

Orion

(g)

Mars

Ans

A

B

(i) Inner Planets (g), (e) Mars, Earth
(ii) Outer Planets (a) Saturn
(iii) Constellation (c), (f) Great Bear, Orion
(iv) Satellite of the earth (d) Moon

Q.161 In which part of the sky can you find Venus if it is visible as an evening star?

Ans

Venus (evening star) can be seen in the western sky after sunset.

Q.162 Name the largest planet of the solar system.

Ans

Jupiter is the largest planet of the solar system.

Q.163 What is a constellation? Name any two constellations.

Ans

A constellation is a group of stars that forms a recognisable pattern in the sky. The two very important constellations are Ursa Major and Orion.

Q.164 Draw sketches to show the relative positions of prominent stars in

(a) Ursa Major and (b) Orion

Ans

(a) Ursa Major: It looks like a big dipper. There are three bright stars in the handle and four stars in the bowl of the dipper.

(b) Orion: It looks like a hunter. Three bright stars appear in the belt, while five bright stars are arranged in the form of a quadrilateral.

Q.165 Name two objects other than planets which are members of the solar system.

Ans

  1. Asteroids: These are the collection of a large number of small objects, gases and dust revolving around the Sun. They cover a large gap between the orbits of Mars and Jupiter.
  2. Meteors: They are small celestial objects that are seen as bright streaks of light in the sky. They burnt out on entering the Earth’s atmosphere because of the heat produced by friction. This results in bright streaks in the sky.

Q.166 Explain how you can locate the Pole Star with the help of Ursa Major.

Ans

The Pole star can be located with the help of Big Dipper constellation. In Big Dipper constellation, the bowl has four bright stars. Now, trace an imaginary straight line towards the Northern direction connecting two stars at the end of this bowl. This imaginary dotted line joins to a star known as the Pole Star.

Q.167 Do all the stars in the sky move? Explain.

Ans

The stars do not move in the sky. However, the stars seem to move in the sky for an observer on the Earth. It is because the Earth rotates from west to east on its axis. Hence, all the stars in the sky appear to move from east to west except Pole star as it is located above the axis of rotation of the Earth in the north direction.

Q.168 Why is the distance between stars expressed in light years? What do you understand by the statement that a star is eight light years away from the Earth?

Ans

The distance between stars or distance of a star from the Earth is so large that it is inconvenient to express these distances in kilometers. Therefore, these large distances are measured in light years. One light year is equal to the distance travelled by light in one year and is equal to 9.46 × 1012 km. If a star is located eight light years away from the Earth, it means distance between the star and the Earth is = 8 × (9.46 × 1012) km or 7.6 × 1013 km away from the Earth.

Q.169 The radius of Jupiter is 11 times the radius of the Earth. Calculate the ratio of the volumes of Jupiter and the Earth. How many Earths can Jupiter accomodate?

Ans

Earth and Jupiter can be assumed as two spheres. Let, radius of the Earth = r 1 radius of the Jupiter = r 2 As radius of Jupiter is 11 times the radius of Earth, Hence, r 2 = 11 r 1 Volume (V) of a sphere of radius r is given as, V = 4 3 π r 3 Volume of Earth, V 1 = 4 3 π r 1 3 (i) Volume of Jupiter, V 2 = 4 3 π r 2 3 = 4 3 π ( 11 r 1 ) 3 (ii) On dividing equation (ii) by (i), V 2 V 1 = 1331( 4 3 π r 1 3 ) 4 3 π r 1 3 =1331 or, V 2 = 1331 V 1 Hence, Jupiter can accommodate 1331 Earths. 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Q.170 Boojho made the following sketch (Fig. 17.29) of the solar system. Is the sketch correct? If not, correct it.

Ans

The sequence of the planets in the solar system from the Sun is: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Hence, the sequence of the planets of solar system is not correctly sketched by Boojho. Moreover, the asteroid belt should be located between the orbits of Mars and Jupiter. The correct sketch of the solar system is shown in the figure given below.

Q.171 Suppose you are in a dark room. Can you see objects in the room? Can you see object outside the room? Explain.

Ans

If we are in a dark room, we cannot see the objects kept inside the room. On the other hand, objects outside the room can be seen. An object becomes visible when light reaches our eye after being reflected from the object. If there is no light in the room, then the objects inside the room will not reflect any light. Hence, we cannot see in a dark room. If there is light present outside the room, then objects placed outside the room can be seen.

Q.172 Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?

Ans

When all parallel rays reflected from a smooth, polished and regular surface are parallel, then the reflection is known as a regular reflection . Clear images are formed by regular reflection. Water and mirror are some of the examples of smooth surface.

If all parallel rays reflected from an irregular surface are not parallel, then the reflection is known as an irregular or diffused reflection. It is caused by irregularities in the reflecting surface and we see hazy image or no image at all. A cardboard and a wall are some of the examples of irregular surface.

In diffused reflection, each ray obeys the laws of reflection. Therefore, laws of reflections are not violated.

Q.173 Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.

(a) Polished wooden table
(b) Chalk powder
(c) Cardboard surface
(d) Marble floor with water spread over it
(e) Mirror
(f) Piece of paper

Ans

(a) Polished wooden table (Regular reflection)
Explanation: A polished wooden table acts as a smooth surface. Hence, reflections from the polished table will be regular.

(b) Chalk powder (Diffused reflection)
Explanation: Chalk powder is not an example of smooth surface. Therefore, diffused reflection will take place from chalk powder.

(c) Cardboard surface (Diffused reflection)
Explanation: Cardboard surface is also an example of an irregular surface. Hence, diffused reflection will take place from a cardboard surface.

(d) Marble floor with water spread over it (Regular reflection)
Explanation: Water makes marble surface smoother. Hence, regular reflection will take place from this surface.

(e) Mirror (Regular reflection)
Explanation: Mirror has a smooth surface. Therefore, it will give a regular reflection.

(f) Piece of paper (Diffused reflection)
Explanation: A piece of paper has many irregularities on its surface. Hence, it will give a diffused reflection.

Q.174 State the laws of reflection.

Ans

There are two laws of reflection:

  1. The angle of reflection is always equal to the angle of incidence.
  2. The incident ray, the reflected ray and the normal to the reflective surface at the point of incidence all lie in the same plane.

Q.175 Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.

Ans

Choose a place in the room where light is very dim. Place a plane mirror on the table. Take a paper sheet and make a small hole in its centre. Hold the sheet normal to the table. Take another sheet and place it on the table in contact with the vertical mirror. Now, draw a normal line on the second sheet from the mirror. Light a torch on the mirror through the small hole in such a way that the ray of light falls on the normal at the bottom of the mirror. When the ray from this hole is incident on the mirror, it gets reflected in a certain direction. Now, it can be observed that the incident ray, reflected ray and the normal to the mirror at the point of incidence all lie in the same plane.

Q.176 Fill in the blanks in the following.

(a) A person 1 m in front of a plane mirror seems to be ___ m away from his image.
(b) If you touch your ___ ear with your right hand in front of a plane mirror, it will be seen in the mirror that your right ear is touched with ____.
(c) The size of the pupil becomes ___ when you see in dim light.
(d) Night birds have ___ cones than rods in their eyes.

Ans

(a) A person 1 m far from a plane mirror seems to be 2 m away from his image.
Explanation: Object distance and image distance are the same from a plane mirror. Therefore, the image distance = 1 + 1 = 2 m away from the person.

(b) If you touch your left ear with your right hand in front of a plane mirror, it will be seen in the mirror that your right ear is touched with your left hand.
Explanation: It is because lateral inversion.

(c) The size of the pupil becomes large when you see in dim light.
Explanation: It is because the amount of light entering into the eye becomes very less. Hence, to increase the amount of light, the pupil expands.

(d) Night birds have less cones than rods in their eyes.
Explanation: Night birds cannot see in the day but can see in the night. They have a few cones and a large number of rod cells on their retina.

Q.177 Angle of incidence is equal to the angle of reflection.

(a) Always
(b) Sometimes
(c) Under special conditions
(d) Never

Ans

The correct option is (a).

Explanation: According to first law of reflection, the angle of incidence is always equal to the angle of reflection.

Q.178 Image formed by a plane mirror is

(a) virtual, behind the mirror and enlarged.
(b) virtual, behind the mirror and of the same size as the object.
(c) real at the surface of the mirror and enlarged.
(d) real, behind the mirror and of the same size as the object.

Ans

The correct option is (b).

Explanation: The image formed by a plane mirror is of the same size as the object and is formed behind the mirror. The image cannot be obtained on a screen. Therefore, it is a virtual image.

Q.179 Describe the construction of a kaleidoscope.

Ans

Take three rectangular mirror strips each about 15 cm long and 4 cm wide. Group them with their long edges together to form a prism. Place and fix them inside a circular cardboard tube or a tube made up of a thick chart paper. Note that the tube must be slightly longer than the mirror strips. Cover one end of the cardboard tube by a cardboard disc. This disc must have a hole at its centre through which we can see. Paste a piece of transparent plastic sheet under the disc in order to make it durable. Fix a circular plane glass plate at the other end of the tube. Place some small pieces of coloured glass on this glass plate. Cover this end of the tube with the ground glass plate and keep enough space for the coloured glass pieces to move. The kaleidoscope is ready. If we look through the hole of the kaleidoscope, beautiful image can be seen.

Q.180 Draw a labelled sketch of the human eye.

Ans

The different parts of the human eye have shown here.

Q.181 Gurmit wanted to perform Activity 16.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher’s advise?

Ans

For human eyes, laser light is dangerous as it’s intensity is very high. It can harm to the retina. Therefore, it is advisable not to look at a laser beam directly.

Q.182 Explain how you can take care of your eyes.

Ans

The given points should be taken into account to protect our eyes:

  1. Do not view the sun or the powerful lights directly.
  2. When advised by doctor, use suitable and proper spectacles.
  3. Too little and too much light is harmful for eyes. Little light leads to eyestrain and headaches while too much light like light from the sun, a powerful lamp or a laser torch can damage the retina.
  4. We should never rub our eyes. When the particles of dust go into our eyes, then we should wash our eyes with clean water or else we should visit to a doctor.
  5. Maintain a distance of at least 25 cm between the book and your eyes while reading.

Q.183 What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?

Ans

According to the law of reflection, the angle of incidence is equal to the angle of reflection. Let, Angle of incident = i Angle of reflection = r As per law of reflection, i = r ∵ i + r = 90° or, i + i = 90° or, i = 90° 2 =45° Therefore, both, angle of incident and angle of reflection will be 45°.

Q.184 How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?

Ans

When two mirrors are placed parallel to each other, then infinite numbers of images are formed due to multiple reflections between the mirrors.

Q.185 Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 16.19. Draw the reflected ray from the second mirror.

Ans

Here, the first law of reflection can be used to obtain the path of reflected light. The given ray of light will reflect from the second mirror at an angle 60°.

Q.186 Boojho stands at A just on the side of a plane mirror as shown in Fig.16.20. Can he see himself in the mirror? Also can he see the image of objects situated at P, Q and R?

Ans

A plane mirror always forms a virtual image behind the mirror and the image distance is equal to the object distance. Boojho cannot see his image because the length of the mirror is too short on his side. Though, he can see the objects placed at points P and Q, but cannot see the object placed at point R.

Q.187 (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21).
(b) Can Paheli at B see this image?
(c) Can Boojho at C see this image?
(d) When Paheli moves from B to C, where does the image of A move?

Ans

(a) Image (A’) of the object (A) is formed behind the mirror. The distance of the image from the mirror is equal to the distance of A from the mirror.
(b) Yes. Paheli can see this image at B.
(c) Yes. Boojho can see this image at C.
(d) Image (A’) of the object at A remain at the same position when Paheli moves from B to C.

Q.188 Fill in the blanks.

(a) Most liquids that conduct electricity are solutions of ______________, ______________ and ______________.
(b) The passage of an electric current through a solution causes ______________ effects.
(c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the ______________ terminal of the battery.
(d) The process of depositing a layer of any desired metal on another material by means of electricity is called ______________.

Ans

(a) Most liquids that conduct electricity are solutions of acid, bases and salts.
Explanation: The solutions of acids, bases or salts can conduct electricity.

(b) The passage of an electric current through a solution causes chemical effects.
Explanation: The solution decomposes into positive and negative ions when electric current passes through it.

(c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the negative terminal of the battery.
Explanation: On passing an electric current through a copper sulphate solution, the solution decomposes into positively charged copper ions and negatively charged sulphate ions. The positively charged copper ions get attracted towards the plate which is connected to the negative terminal of the battery.

(d) The process of depositing a layer of any desired metal on another material by means of electricity is called electroplating.

Q.189 When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason?

Ans

The deflection in the compass needle proves that current is flowing through the circuit. It shows that the solution is a conducting solution.

Q.190 Name three liquids, which when tested in the manner shown in Fig.14.9, may cause the magnetic needle to deflect.

Ans

The liquids such as lemon juice, salt water and vegetable oil allow electricity to pass through them. Therefore, these liquids can be used in the beaker shown here.

Q.191 The bulb does not glow in the setup shown in Fig.14.10. List the possible reasons. Explain your answer.


Ans

There may be two reasons for this.

(i) The battery connected in the circuit has insufficient energy to generate electricity.
(ii) Liquid kept in the beaker is non-conducting. In this case, current would not be able to pass through the liquid.

Q.192 A tester is used to check the conduction of electricity through two liquids, labelled A and B. It is found that the bulb of the tester glows brightly for liquid A while it glows very dimly for liquid B. You would conclude that

(i) liquid A is a better conductor than liquid B.

(ii) liquid B is a better conductor than liquid A.

(iii) both liquids are equally conducting.

(iv) conducting properties of liquid cannot be compared in this manner.

Ans

The correct option is (i).

Explanation: The magnitude of current flowing through a conducting solution depends on the conductivity of the solution. The solution having more conductivity will allow more current to pass through it.

Q.193 Does pure water conduct electricity? If not, what can we do to make it conducting?

Ans

No,Pure water does not conduct electricity because it does not contain salt. When some common salt is added to the pure water, it can conduct electricity.

Q.194 In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explain why they do this.

Ans

Water can act as a conductor of electricity. In case of a fire, if the electrical supply for the area is not shut off and water is poured over electrical appliances, then the fireman can get electric shocks.

Q.195 A child staying in a coastal region tests the drinking water and also the seawater with his tester. He finds that the compass needle deflects more in the case of seawater. Can you explain the reason?

Ans

Sea water is more salty than the drinking water. Therefore, it is more conducting than the drinking water. Due to this, the compass needle deflects more in the case of seawater than in the drinking water.

Q.196 Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.

Ans

No, During heavy downpour, it is not safe to repair electrical appliances outdoors. Rain water can conduct electricity as it contains dissolved salts. Hence, the electrician may get electrical shocks while working outdoors during rain.

Q.197 Paheli had heard that rainwater is as good as distilled water. So she collected some rainwater in a clean glass tumbler and tested it using a tester. To her surprise she found that the compass needle showed deflection. What could be the reasons?

Ans

Rain water acts as conducting solution as it contains dissolved salts. On the other hand, there are no dissolved salts present in the distilled water. Therefore, rain water can allow electricity to pass through it.

Q.198 Prepare a list of objects around you that are electroplated.

Ans

  1. In gold-plated ornaments, a fine layer of gold is deposited on the silver ornaments.
  2. Chromium plating is done on different parts of vehicles to make them shiny.
  3. To protect iron from corrosion, iron is coated with zinc layer.

Q.199 The process that you saw in Activity 14.7 is used for purification of copper. A thin plate of pure copper and a thick rod of impure copper are used as electrodes. Copper from impure rod is sought to be transfered to the thin copper plate. Which electrode should be attached to the positive terminal of the battery and why?

Ans

Copper ions have positive charges. Hence, they are attracted towards the plate which is connected to the negative terminal of the battery. As copper ions are transferred to the thin copper plate, this thin pure copper plate should be connected to the negative terminal of the battery. As a result, impure copper rod should be connected to the positive terminal of the battery.

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