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Melting, Boiling and Evaporation


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latent-heat


MCQ Questions

1. Which of the following types of energy remains constant during a change of state?
a. internal energy
b. potential energy
c. kinetic energy
d. heat energy

2. The energy required for a change of state is called
a. chemical energy
b. state energy
c. latent energy
d. heat energy

3. If a substance expands on melting, increased pressure acting on it will
a. not change its melting point
b. increase its melting point
c. decrease its melting point
d. condense the substance

4. The reversed process of melting is
a. condensing
b. freezing
c. sublimation
d. burning

5. The boiling point of water in a pressure cooker is raised by
a. increasing the volume of water in the cooker
b. increasing the internal volume of the cooker
c. increasing the pressure in the cooker
d. increasing the pressure outside the cooker

6. If a sample of water boils at a temperature above 100oC, the water
a. is pure
b. contains impurities
c. boils at a pressure lower than normal atmospheric pressure
d. boils in vacuum

7. A liquid evaporates at
a. temperatures above its boiling point
b. its boiling point
c. temperatures below its boiling point
d. temperatures above and below its boiling point

8. Which of the following liquids is used as the cooling agent in a household fridge?
a. alcohol
b. ether
c. freon
d. liquid hydrogen

9. The specific latent heat of fusion is absorbed by a substance when it changes from
a. a liquid to a gas
b. a gas to a liquid
c. a solid to a liquid
d. a liquid to a solid

10. The latent heat of fusion supplied to a substance is used
a. to keep its particles in their fixed positions
b. by particles to break away from their fixed positions
c. to increase the average speed of its particles in their random motion
d. to decrease the average speed of its particles in their random motion

MCQ Answers

1. c
2. c
3. b
4. b
5. c
6. b
7. c
8. c
9. c
10. b

Structured Questions

1a. Name the following processes
i. a solid changes into a liquid
ii. a liquid changes into a solid
iii. a liquid changes into a gas at a particular temperature
iv. a liquid changes into a gas at ordinary room temperature
v. a gas changes into a liquid

b. What would you expect the boiling point of water to be if its freezing point is less than 0oC?

c. Describe how the melting point of ice is affected by
i. presence of sugar
ii. altitude

Solution

1ai. melting
aii. freezing
aiii. boiling
aiv. evaporation
av. condensation

b. more than 100oC
ci. lowered
cii. increased

2a. Explain why we feel cooler when wind blows over us and evaporates the sweat ob our skins

b. Using your knowledge of the kinetic theory, explain why evaporation produces a cooling effect

c. An immersion heater with an output of 240W is embedded in a large block of ice at 0oC. After the heater has been switched on for 11 min and 12 s, it is found that 480g of ice has melted into water at 0oC. Calculate the specific latent heat of fusion of ice.

d. Why is the specific latent heat of vaporization of a substance much greater than its specific latent heat of fusion?

Solution

2a. Evaporation of sweat which is a liquid produces a cooling effect. Furthermore, the rate of evaporation is increased by the movement of air

2d. 240 x 672 = 0.48 l
l = 336000 j kg-1

3. A 200W heater is used to melt ice at 0oC in a filter funnel. After 300s, the mass of water collected is 0.188kg. If 0.088kg of the ice melted purely due to heat taken in from the surroundings (and not from the heater), calculate the specific latent heat of fusion of ice.

Solution

3. 200 x 300 = (0.188 - 0.088) l
60 000 = 0.1 l
l = 600 000 Jk/kg

4. A 100g packet of frozen peas at 0oC is taken from the cold compartment of a fridge. After 20min, the ice has completely melted and produced 5g of water
a. assuming that the peas and the water are still at 0oC, calculate the rate at which heat has been gained from the surroundings to melt the ice. Assume that the specific latent heat of melting ice is 340 J/g
bi. suggest why the rate of gain of heat might have been larger than the value you have calculated
bii. if the peas and water are left for several more minutes, the rate of gain of heat decreases. Suggest why this is so.

Solution

4a. amount of heat absorbed by ice = ml = 5 x 340 = 1700J
rate of heat gained = 1700/20 = 85J/min

bi. heat must be conducted from the packet to the contents. This means the packet must be at a higher temperature and would need to absorb some heat from the surroundings
bii. this is because the difference in temperature between the system and the atmosphere has reduced. The rate of heat gain depends on this temperature difference


5.
The figure shows a graph of temperature against time obtained by cooling a vapour. Explain how the energy is given out in the portions of the graph labelled
a. AB
b. BC
c. CD

d. DE


Solution

5a. latent heat of vaporization is given out when the vapour condenses into a liquid
b. heat is given out when the hot liquid cools
c. latent heat of fusion is given out when the liquid freezes and becomes solid
d. heat is given out when the hot solid cools


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