CBSE Important Questions Class 9 Science Chapter 1 – Matter in Our Surroundings
Class 9 Science Chapter 1 Matter in Our Surroundings is the first chapter of the CBSE Class 9 Science syllabus. It explains what matter is, the characteristics of its particles, and the three states of matter: solid, liquid and gas. Students learn how matter changes from one state to another on heating and cooling, the meaning of melting point and boiling point, latent heat, evaporation and the cooling effect it produces.
These CBSE important questions for Class 9 Science Chapter 1 cover the factors that affect the rate of evaporation, why sweating cools the body, the three states of matter and their properties, latent heat of fusion, converting between the Celsius and Kelvin scales, and sublimation. Every question has a clear step-by-step answer in simple English with figures, so students can practise on their own, parents can check the answers, and teachers can use them for class tests.
Q1. Which of the following factors do not affect the rate of evaporation?
I: Pressure of the liquid
II: Humidity of the surrounding air
III: Depth of the liquid
IV: Surface area of the liquid
(a) I and II
(b) II and III
(c) I and III
(d) III and IV
Answer: (c) I and III
Evaporation happens only at the surface of a liquid. So the depth of the liquid makes no difference, and the pressure of the liquid also has no real effect under normal conditions. Humidity and surface area do affect the rate, so II and IV are not the answer.
Q2. What are the factors on which evaporation depends?
Answer: Evaporation depends on four main factors:
A. Surface area: the larger the surface, the faster the evaporation.
B. Temperature: at a higher temperature the particles get more energy, so evaporation is faster.
C. Wind velocity: moving air carries the vapour away, so evaporation is faster.
D. Humidity: if the air already has a lot of water vapour, evaporation becomes slower.
Q3. Which of the following will have zero effect on the process of evaporation?
(a) surface area
(b) humidity
(c) pressure
(d) temperature
Answer: (c) pressure
Surface area, humidity and temperature all change how fast a liquid evaporates. Under ordinary conditions, the pressure of the liquid has practically no effect on evaporation.
Q4. The factor which makes the rate of evaporation faster is
(a) increase in temperature
(b) decrease in surface area
(c) increase in humidity
(d) decrease in wind speed
Answer: (a) increase in temperature
When the temperature rises, the particles of the liquid gain more kinetic energy and escape into the air more easily. The other three options all slow evaporation down.
Q5. Choose the factor(s) which affect(s) the rate of evaporation.
I. Humidity
II. Surface area
III. Pressure
IV. Wind speed
(a) I and II
(b) II and III
(c) III and IV
(d) I, II and IV
Answer: (d) I, II and IV
Humidity, surface area and wind speed all change the rate of evaporation. Pressure has no significant effect, so III is left out.
Q6. Read the Assertion and Reason carefully and choose the correct option.
Assertion (A): Perspiration keeps our body cool.
Reason (R): Sweat evaporates by absorbing heat from the body and we feel cool.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Sweat is mostly water. To change from liquid to vapour it needs heat, which it takes from our body (this is called latent heat of vaporisation). As the body loses this heat, we feel cool. So the Reason correctly explains the Assertion.
Q7. Pick the conditions under which evaporation of water increases.
(a) Increase in surface area and decrease in temperature
(b) Increase in temperature and decrease in surface area
(c) High humidity and decrease in surface area
(d) Increase in surface area and increase in temperature
Answer: (d) Increase in surface area and increase in temperature
Both of these make evaporation faster. A larger surface lets more particles escape, and a higher temperature gives the particles more energy. All the other options include at least one factor that slows evaporation down.
Q8. Define matter. Name the three states of matter and give one example of each.
Answer: Anything that has mass and occupies space is called matter.
The three states of matter are:
1. Solid: has a fixed shape and a fixed volume. Example: ice, stone.
2. Liquid: has a fixed volume but no fixed shape; it takes the shape of the container. Example: water, milk.
3. Gas: has neither a fixed shape nor a fixed volume; it fills the whole container. Example: oxygen, water vapour.
Q9. What is latent heat of fusion? Why does ice at 0 °C cool a drink better than water at 0 °C?
Answer: The latent heat of fusion is the heat needed to change 1 kg of a solid into liquid at its melting point, without any change in temperature.
Ice at 0 °C cools a drink better than water at 0 °C because the ice must first absorb its latent heat of fusion (about 3.34 × 10⁵ J/kg) to melt. It takes this extra heat from the drink, so the drink becomes much colder. Water at 0 °C has already absorbed that heat, so it cannot take as much heat from the drink.
Q10. Give reasons:
(a) A gas fills completely the vessel in which it is kept.
(b) We can easily move our hand in air but to do the same through a solid block of wood we need a karate expert.
(c) A wooden table should be called a solid.
Answer: (a) In a gas the particles are very far apart and move about freely at high speed in all directions. So they spread out and fill the whole vessel.
(b) In air the particles are far apart and the force of attraction between them is very weak, so our hand moves through easily. In wood the particles are packed very close with strong forces between them, so a large force is needed to separate them.
(c) A wooden table has a fixed shape, a fixed volume, a high density and it is rigid. These are the properties of a solid, so it is called a solid.
Q11. Convert the following temperatures to the Celsius scale:
(a) 300 K
(b) 573 K
Convert the following temperatures to the Kelvin scale:
(c) 25 °C
(d) 373 °C
Answer: To change Kelvin into Celsius, subtract 273. To change Celsius into Kelvin, add 273.
(a) 300 − 273 = 27 °C
(b) 573 − 273 = 300 °C
(c) 25 + 273 = 298 K
(d) 373 + 273 = 646 K
Q12. What is sublimation? Give two examples. How is it different from evaporation?
Answer: Sublimation is the change of a solid directly into a gas (vapour) on heating, without first becoming a liquid. The reverse change, gas straight to solid, is called deposition.
Examples: camphor, naphthalene balls, dry ice (solid carbon dioxide) and ammonium chloride.
Difference: In sublimation a solid changes directly into vapour, and it usually needs heating. In evaporation a liquid changes into vapour, and it happens slowly at any temperature below the boiling point, only from the surface.
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Q.1 Samantha kept a raisin in solution X. After 3 hours she observed that the size of the raisin had increased. What can you infer about the solution X
A. Solution X is hypotonic
B. Solution X is hypertonic
C. Solution X is isotonic
D. Solution X is a mixture of sugar and water
Marks:1
Ans
Solution X is hypotonic
Q.2 The plasma membrane is called a selectively permeable membrane. Justify the statement.
Marks:2
Ans
The plasma membrane allows the entry and exit of only some selected materials in and out of the cell. Hence, it is a selectively permeable membrane.
Q.3 Observe the given picture carefully and answer the following questions:
(i) Categorise the given cell either into plant cell or animal cell and specify any one basis for it.
(ii) Out of A, B, C and D, which one is regarded as Power house of the cell
(iii) Write any one function of the cell organelle labeled as C.
Marks:3
Ans
(i) The depicted cell is an animal cell since it does not contain a large vacuole occupying most of the cell volume as the plant cells usually possess.
(ii) D is the mitochondrion which is also regarded as the Power house of the cell. It is mainly because this cell organelle is responsible for the generation of ATP.
(iii) C is the Golgi apparatus whose function is to package the molecular materials and deliver them to the intra-cellular targets.
Q.4 Riya used eosin to stain onion peel cells, but the cells did not take up the stain. On the other hand, cells took up the stain when she used safranin instead of eosin. Why is safranin predominantly used for the staining of onion peel cells
Marks:3
Ans
Safranin is a basic dye. Plant cells typically possess negatively charged cell walls. The positive colouring molecules present in basic dyes stick to the cell walls and stain them. Eosin, picric acid, and acid fuchsine are acidic dyes.