Vidaara.orgClass 9 · Chemistry
CodeVID-C9-CHEM-01-T2-01
Assignment — Change of State & Latent Heat
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Show all working for the numerical questions. For full solutions, raise your doubts with your teacher.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
The melting point of ice at normal pressure is:
- A.100°C
- B.0°C
- C.273°C
- D.-273°C
2.
373 K on the Celsius scale is:
- A.0°C
- B.100°C
- C.273°C
- D.646°C
3.
Which of these sublimes?
- A.common salt
- B.camphor
- C.iron
- D.sugar
4.
The hidden heat absorbed during a change of state is called:
- A.specific heat
- B.latent heat
- C.kinetic energy
- D.heat capacity
5.
Lowering the surrounding pressure on a liquid will:
- A.raise its boiling point
- B.lower its boiling point
- C.not affect it
- D.freeze it
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
Convert 25°C into Kelvin and 350 K into Celsius.
7.
Define latent heat of vaporisation.
8.
Why does the temperature not change while a substance is boiling?
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
Distinguish between melting and sublimation with one example of each.
10.
Explain how the boiling point of water is affected (a) in a pressure cooker and (b) on a high mountain.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
Draw and describe the heating curve obtained when ice at -10°C is heated steadily to steam. Explain the two flat portions of the curve.
Answer Key
Section A — Multiple Choice Questions
- (B) 0°C
- (B) 100°C
- (B) camphor
- (B) latent heat
- (B) lower its boiling point
Section B — Short Answer (2 marks)
- 25°C = 25 + 273 = 298 K; 350 K = 350 - 273 = 77°C.
- It is the heat energy required to change 1 kg of a liquid into vapour at its boiling point without any rise in temperature.
- The heat supplied (latent heat of vaporisation) is used to break the forces of attraction between particles, not to raise their kinetic energy, so the temperature stays constant.
Section C — Short Answer (3 marks)
- Melting is the change of a solid to a liquid at its melting point (ice to water at 0°C). Sublimation is the change of a solid directly to vapour without becoming a liquid (camphor or dry ice turning to vapour).
- (a) In a pressure cooker the pressure is higher than normal, so the boiling point rises above 100°C and food cooks faster. (b) On a high mountain the atmospheric pressure is lower, so the boiling point falls below 100°C and cooking takes longer.
Section D — Long Answer (5 marks)
- The curve rises as the ice warms to 0°C, then stays flat at 0°C while the ice melts (first plateau), rises again as the water warms to 100°C, stays flat at 100°C while the water boils (second plateau), then rises as the steam is heated further. The two flat portions occur because, during melting and boiling, the heat supplied is absorbed as latent heat (of fusion and of vaporisation) to break the forces between particles, so the temperature remains constant until the change of state is complete.
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