Vidaara.orgClass 9 · Chemistry
CodeVID-C9-CHEM-01-T3-01
Assignment — Evaporation & Factors Affecting It
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.
- Give reasons for your answers wherever asked. For full solutions, raise your doubts with your teacher.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
Evaporation is a:
- A.bulk phenomenon
- B.surface phenomenon
- C.type of boiling
- D.type of freezing
2.
Evaporation is fastest when humidity is:
- A.high
- B.low
- C.100%
- D.constant
3.
Which of these will cool fastest by evaporation?
- A.water in a bottle
- B.water in a wide tray
- C.water in a narrow tube
- D.water in a sealed jar
4.
The cooling produced by sweating is due to:
- A.boiling of sweat
- B.evaporation of sweat
- C.freezing of sweat
- D.sublimation of sweat
5.
Increasing the wind speed over a liquid will:
- A.decrease evaporation
- B.increase evaporation
- C.stop evaporation
- D.have no effect
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
Define evaporation and state how it differs from boiling.
7.
Why does our palm feel cold when we put perfume on it?
8.
State how humidity affects the rate of evaporation.
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
Explain, with the particle model, why evaporation causes cooling.
10.
List the four factors affecting the rate of evaporation and give one everyday example of each.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
Describe an activity to show that evaporation causes cooling, and use it to explain why people sprinkle water on the roof or ground on a hot summer evening.
Answer Key
Section A — Multiple Choice Questions
- (B) surface phenomenon
- (B) low
- (B) water in a wide tray
- (B) evaporation of sweat
- (B) increase evaporation
Section B — Short Answer (2 marks)
- Evaporation is the change of a liquid into vapour below its boiling point and occurs only at the surface; boiling occurs throughout the bulk of the liquid at a fixed temperature (the boiling point).
- Perfume is volatile and evaporates quickly, absorbing latent heat from the skin, which lowers the skin temperature and makes the palm feel cold.
- When humidity is high, the air already holds a lot of water vapour and is near saturation, so it can accept little more vapour and the rate of evaporation decreases.
Section C — Short Answer (3 marks)
- Particles in a liquid have different kinetic energies. During evaporation, the fastest, most energetic particles escape from the surface. The particles left behind have a lower average kinetic energy, so the temperature of the liquid falls. To continue evaporating, the liquid absorbs heat (latent heat) from its surroundings, producing a cooling effect.
- Surface area (tea cools faster in a saucer than a cup); temperature (clothes dry faster on a hot day); humidity (clothes dry slowly on a rainy day); wind speed (clothes dry faster on a windy day).
Section D — Long Answer (5 marks)
- Activity: Put a few drops of a volatile liquid such as acetone, ether or spirit on your palm and observe. As the liquid evaporates, the palm feels distinctly cold because the liquid absorbs latent heat from the skin to change into vapour. This shows evaporation causes cooling. On a hot summer evening, sprinkling water on a roof or courtyard works the same way: the water evaporates and absorbs heat (latent heat) from the hot surface and the surrounding air, lowering their temperature and making the surroundings feel cooler.
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