Exercises

Question 1

Convert the following temperatures to the Celsius scale:

(a) 293 K

(b) 470 K

Answer:

Formula used:

$${}^{\circ }C=K-273$$

• (a) 293 K = 293 − 273 = 20°C

• (b) 470 K = 470 − 273 = 197°C

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Question 2

Convert the following temperatures to the Kelvin scale:

(a) 25°C

(b) 373°C

Answer:

Formula used:

$$K{=}^{\circ }C+273$$

• (a) 25°C = 25 + 273 = 298 K

• (b) 373°C = 373 + 273 = 646 K

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Question 3

Give reason for the following observations:

(a) Naphthalene balls disappear with time without leaving any solid.

Answer:
Naphthalene undergoes sublimation, that is, it changes directly from solid to gas without changing into liquid. Hence, it disappears without leaving any solid residue.

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(b) We can get the smell of perfume sitting several metres away.

Answer:
Perfume particles diffuse rapidly in air due to continuous random motion of gas particles. This process is called diffusion, which allows the smell to spread over long distances.

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Question 4

Arrange the following substances in increasing order of forces of attraction between the particles:

Water, sugar, oxygen

Answer:

Oxygen < Water < Sugar

Explanation:

• Oxygen (gas): Least force of attraction

• Water (liquid): Moderate force of attraction

• Sugar (solid): Maximum force of attraction

Question 5

What is the physical state of water at—
(a) 25°C (b) 0°C (c) 100°C

Answer:

• (a) 25°C: Liquid state

• (b) 0°C: Both solid (ice) and liquid (water) may exist

• (c) 100°C: Both liquid water and gas (steam) may exist

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Question 6

Give two reasons to justify—

(a) Water at room temperature is a liquid.

Answer:

1. Water has a fixed volume but no fixed shape.

2. It can flow and takes the shape of the container.

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(b) An iron almirah is a solid at room temperature.

Answer:

1. It has a fixed shape and fixed volume.

2. It is rigid and not easily compressible.

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Question 7

Why is ice at 273 K more effective in cooling than water at the same temperature?

Answer:

Ice at 273 K absorbs latent heat of fusion from the surroundings to change into water.
This extra absorption of heat makes ice more effective in cooling than water at the same temperature.

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Question 8

What produces more severe burns, boiling water or steam?

Answer:

Steam produces more severe burns than boiling water because:

• Steam contains latent heat of vaporisation

• When steam condenses on skin, it releases extra heat

• This causes more damage than boiling water

Question 9

Name A, B, C, D, E and F in the following diagram showing change in its state.

(The diagram shows interconversion of solid, liquid and gas by changing temperature and pressure.)

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Answer

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Explanation 

• Melting (A): Change of solid into liquid by heating

• Vaporisation (B): Change of liquid into gas by heating

• Condensation (C): Change of gas into liquid by cooling

• Freezing (D): Change of liquid into solid by cooling

• Sublimation (E): Direct change of solid into gas

• Deposition (F): Direct change of gas into solid