NCERT Solutions for Class 7 Science Chapter 6 : Physical And Chemical Changes

Q:

Anaerobic bacteria digest animal waste and produce biogas (Change – A). The biogas is then burnt as fuel (Change – B). The following statements pertain to these changes. Choose the correct one.

(i) Process – A is a chemical change.

(ii) Process – B is a chemical change.

(iii) Both processes A and B are chemical changes.

(iv) None of these processes is a chemical change.

A:

The correct option is (iii). Both processes A and B are chemical changes.

Q:

The gas we use in the kitchen is called liquefied petroleum gas (LPG). In the cylinder it exists as a liquid. When it comes out from the cylinder it becomes a gas (Change – A) then it burns (change – B). The following statements pertain to these changes. Choose the correct one.

(i) Process – A is a chemical change.

(ii) Process – B is a chemical change.

(iii) Both processes A and B are chemical changes.

(iv) None of these processes is a chemical change.

A:

The correct option is (ii). Process – B is a chemical change.

Q:

Explain how painting of an iron gate prevents it from rusting.

A:

The presence of both oxygen (air) and water (moisture) is essential for rusting to take place.When an iron gate is painted, oxygen and water do not come in contact with iron. Hence, rusting is prevented.

Q:

Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

A:

Cutting of wood into small pieces is a physical change because no new substance is formed and only the shape and size of wood changes.When we burn wood, carbon dioxide along with some other new substances is formed. Therefore, burning of wood is a type of chemical change.

Q:

How would you show that setting of curd is a chemical change?

A:

Setting of curd is type of chemical change as new substance is formed which has different properties from milk. Once curd is formed, milk cannot be re-obtained from it.

Q:

Explain why rusting of iron object is faster in coastal areas than in deserts.

A:

The presence of both oxygen and water is essential for rusting. Greater humidity indicates presence of more amount of water vapours in air, therefore, faster rate of rusting. In coastal areas humidity is high as compared to deserts. Moreover, the presence of salty water also speeds up the process of rusting in coastal areas. Thus, the iron objects rust faster in coastal areas.

Q:

When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain

A:

When baking soda (sodium hydrogencarbonate) is mixed with lemon juice (citric acid), bubbles are produced due to the formation of carbon dioxide and other substances. It is a chemical change. Therefore, we cannot get back the original substances, i.e., lemon juice and baking soda.

Q:

State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.

(a) Cutting a log of wood into pieces is a chemical change. (True/False)

(b) Formation of manure from leaves is a physical change. (True/False)

(c) Iron pipes coated with zinc do not get rusted easily. (True/False)

(d) Iron and rust are the same substances. (True/False)

(e) Condensation of steam is not a chemical change. (True/False)

A:

(a) Cutting a log of wood into pieces is a chemical change.    (False)

Corrected statement: Cutting a log of wood into pieces is a physical change.

(b) Formation of manure from leaves is a physical change.   (False)

Corrected statement: Formation of manure from leaves is a chemical change.  

(c) Iron pipes coated with zinc do not get rusted easily.     (True)

(d) Iron and rust are the same substances.  (False)

Corrected statement: Iron and rust are different substances.

(e) Condensation of steam is not a chemical change.   (True)

Q:

Classify the changes involved in the following processes as physical or chemical changes:

(a) Photosynthesis

(b) Dissolving sugar in water

(c) Burning of coal

(d) Melting of wax

(e) Beating aluminium to make aluminiun foil

(f) Digestion of food

A:

(a) Photosynthesis: Chemical change

(b) Dissolving sugar in water: Physical change

(c) Burning of coal: Chemical change

(d) Melting of wax: Physical change

(e) Beating aluminium to make aluminium foil: Physical change

(f) Digestion of food: Chemical change

Q:

Describe how crystals of copper sulphate are prepared.

A:

Crystal of copper sulphate is obtained by the process of crystallisation. For the preparation of crystals of copper sulphate take some water in a beaker and add a few drops of dilute sulphuric acid. Heat the water. When it starts boiling add copper sulphate powder slowly while stirring continuously. Continue adding copper sulphate powder till it stops dissolving. Filter the solution into a china dish and allow it to cool. Do not disturb the solution while it cools. Slowly, the crystals of copper sulphate separate out.

Q:

When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.

A:

When a candle burns, both physical and chemical changes take place.When we burn a candle, the wax melts which can be converted back into solid wax on cooling. This shows that melting of wax is a physical change. Burning of a candle also produces light along with some gases like carbon dioxide. Hence, burning of the wick of a candle is a chemical change.

Cooking food is another example in which both physical and chemical changes take place.

Physical change: appearance of ingredients

Chemical change: new substances are formed

Q:

Fill in the blanks in the following statements:

(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of ________.

(b) The chemical name of baking soda is ________.

(c) Two methods by which rusting of iron can be prevented are _________ and _________.

(d) Changes in which only ________ properties of a substance change are called physical changes.

(e) Changes in which new substances are formed are called _________ changes.

A:

(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of calcium carbonate.

(b) The chemical name of baking soda is sodium hydrogencarbonate.

(c) Two methods by which rusting of iron can be prevented are painting and galvanisation.

(d) Changes in which only physical properties of a substance change are called physical changes.

(e) Changes in which new substances are formed are called chemical changes.

Benefits of NCERT Solutions

Following are some of the benefits of using NCERT Solutions by Extramarks.

1. The NCERT solutions for class 7 are prepared keeping in mind the necessary rules and format issued by the CBSE board thus providing students much-needed preview of the exam pattern of questions and preview of the exam that they are going to face during the final exams.

2. The students can be sure about the quality of the solutions as they are committed by highly experienced and expert teachers. These teachers have years of experience and they make sure that there is no mistake committed when preparing the solutions.

3. The NCERT 7 solutions are provided for the questions that are most likely to appear in the exam thus improving the efficiency of the students by helping them prepare for the important parts of the syllabus.

Tips & Strategies for Class 7 Exam Preparation

1. Start your revision early so you can revise well and grasp all the difficult concepts.

2. Organize your study time by making a timetable as it will help you manage your time accordingly.

3. Take mock tests and solve sample papers as they will help you learn time management during the exam and make you familiar with the exam pattern.

4. Take regular breaks, look after yourself by sleeping properly and eating well.

5. Know your exam well, explore the mock tests, and practice sample papers as much as you can.

Why Choose Extramarks CBSE Class 7 Study Materials?

1. The solutions have been compiled keeping in mind the purpose of scoring high marks in exams thus they have everything that the students need to prepare for exams.
2. The NCERT solutions for class 7 are designed in a clear and simple language complete with diagrams so that the students understand the subject well.
3. The NCERT 7 solutions are accurate thus improving their reliability.
4. The solutions are designed by expert and experienced teachers that help in removing all doubts out of the solutions and improving the credibility of the answers.

Frequently Asked Questions

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Is class 7 Math NCERT book sufficient for prepare for the exams?

Yes, the NCERT book has been designed keeping in mind the pattern of paper that will be presented in the exam to students. The NCERT book is sufficient to study for the exam as it has everything related to the exam and contains all information out of which the questions will be asked in the exam. But if you want to prepare a little bit extra, we suggest that you go through the NCERT class 7 solutions.

When to refer to NCERT solutions for class 7th?

When you are studying the NCERT book for math and other subjects for your class 7 exams, refer to the NCERT class 7 solutions simultaneously so you can see where you are going wrong and what are you getting right in your exercises’ attempt.

Why are the NCERT solutions important for Class 7th?

The class 7th is very crucial as it is among the major middle school classes during which the child’s base is formed in math and science and even in other subjects like English. Thus, it is students must study smartly in this class and ace their exams while getting all the help they need from the NCERT 7 solutions given by Extramarks.

Can I rely on the NCERT solutions for class 7 Math at Extramarks to perform well in the annual exam?

Yes, absolutely. The NCERT solutions provided by Extramarks for class 7 are eloquent and elaborate at best and are thus useful for students studying for the final exams of class 7. The students will have many questions and doubts regarding the NCERT solutions for class 7 and this is why the solutions have been elaborated well to help students understand the answers to exercises.

Do you provide NCERT solutions for Class 7 Math for free?

Yes, the NCERT solutions for Math and other subjects for class 7 are available for free from Extramarks.

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