Book 1 (Chapter 3: Latent Heat)

Question 1

Latent Heat (definition)

Answer 1

energy required to change the state without change in temperature

Question 2

Change of State

Answer 2

Solid–>liquid (fusion - melting)
liquid–>gas (vapourization - boiling)

gas–> liquid (condensation)
liquid–>solid (solidification - freezing)

Question 3

Process of state change

Answer 3

• melting point and boiling point of a material are constant, not affected by its mass and the heating process
  - melting point = freezing point

To determine the state of a substance at temperature
(1) vapour state –> T > B.P.
(2) liquid state –> M.P. < T < B.P.
(3) solid state –> T < M.P.

Question 4

M.P. and B.P. would be affected by:

Answer 4

(1) the atmospheric pressure
(2) the impurity

Question 5

Specific latent heat (Definition)

Answer 5

energy absorbed or released by 1kg of the material during the change of state

Unit: J kg-1

Equation:
E=ml

Question 6

Specific latent heat of fusion is not affected by:

Answer 6

(1) mass of the material
(2) melting point of the material
(3) shape of the material
(4) type of container

Question 7

Specific latent heat of vaporisation is not affected by:

Answer 7

mass of the material, boiling point of the material, type of container

Question 8

Why energy is needed to change the state from solid to liquid (or from liquid to gas) while the temperature is remained unchanged?

Answer 8

Because when the state is changed from solid to liquid, the distance between the particles increases, potential energy increases.

Question 9

Heating curve (solid–>gas)

Answer 9

–>absorbing latent heat of fusion
–>absorbing latent heat of vaporisation

Question 10

Cooling curve (gas–>solid)

Answer 10

–>releasing latent heat of vapourisation
–> releasing latent heat of fusion

Question 11

Specific latent heat (things to memorise)

Answer 11

water–>steam: 2260000J (absorbed/released)

ice–>water: 334000J (absorbed/released)

Question 12

Comparison between the two specific latent heat:

Answer 12

Due to the high value of the specific latent heat of vapourization of water:
- Steam at 373K stores much more internal energy than water at 373K
- Steam causes a more severe burn than boiling water since large amount of latent heat of vapourization is released when steam condenses

Question 13

After the water boils, an egg is placed into the water for cooking. Can the egg be cooked in a shorter time by using high flame after the water has reached the boiling point? Explain briefly.

Answer 13

No! By using high flame, the water temperature still remains at the boiling point of 373K, thus the rate of energy supplied by the water to the egg is the same. Therefore, it takes the same time to cook the egg.

Question 14

Kinetic energy of molecules

Answer 14

• the average kinetic energy of molecules depends on temperature only
• the average speed of molecules is related to the kinetic energy, and thus depends on temperature

Question 15

Potential energy of molecules

Answer 15

• it is changed if and only if the state changes
• When a substance undergoes state change by heating:
  (1) energy is supplied to pull apart the molecules against the attractive forces between water molecules –> this supply of energy is the latent heat and the energy would be stored as the potential energy of molecules
  (2) the kinetic energy of the molecules remain unchanged –> the temperature remains unchanged and the average speed of molecules remain unchanged

potential energy of vapour > potential energy of liquid > potential energy of solid > potential energy of solid

Question 16

Explain the following phenomenon:
(1) Heat is needed to keep water boiling although the temperature of water does not rise.

(2) Steam causes more severe burn than boiling water.

Answer 16

(1): Since energy has to supply to water to pull apart the molecules against their attraction and becomes the potential energy of molecules, thus, latent heat of vaporisation is needed to keep water boiling.

(2): When steam condenses, it fives out large amount of latent heat of vaporisation which causes more severe burn.

Question 17

Conservation of energy

Answer 17

When 2 bodies of different temperatures are put together, heat would flow from the high temperature body to the low temperature body until they are finally at the same temperature, i.e., they are in thermal equlibrium.

Assume there is no heat exchange with the surroundings: energy gained by the cold body = energy lost by the hot body

Question 18

Mixture of melting ice and water

Answer 18

(1) If the ice does not melt completely, the final temperature must be 273K.
(2) If the ice melts completely, then the final temperature of the micture must be higher than or equal to 273K.
(Latent heat gained by the ice during melting + Heat gained by the water melted from ice
= heat lost by the hot water)

Question 19

Mixture of steam and water

Answer 19

(1) If the steam does not condense completely, then the final temperature must be 373K.

(2) If the steam condenses completely, then the final temperature of the mixture must be lower than or equal to 373K.
(Latent heat lost by the steam during condensation + heat lost by the water condensed from steam = heat gained by the cold water)

Question 20

Mixture of ice and steam

Answer 20

• When steam at 373K is mixed with ice at 273K, steam condenses to release latent heat of vaporisation while ice melts to absorb the latent heat of fusion
• the specific latent heat of vaporisation of steam is much greater than the specific latent heat of ice.
• Suppose equal mass of ice at 273K and steam at 373K are mixed in an insulated container: Assume there is not heat loss to the surroundings –> by conservation of energy, heat gained by the ice = heat lost by the steam –> the latent heat released by a small fraction of steam condensed is large enough to melt all the ice and then heat the water to 373K. –> the final product would be a mixture of water and steam at 373K.

Question 21

Evaporation

Answer 21

DEFINITION:
- a liquid changes into vapour at temperatures below the boiling point is called evaporation.
- during evaporation, the liquid still absorbs latent heat of vaporisation to change into vapour.
- note that in evaporation, the water vapour is not visible

Question 22

Daily life examples of evaporation

Answer 22

1. Wet clothes get dry in air.
2. Water in a cup disappears after a few days.
3. Wet hair dries in air.

Question 23

Differences between boiling and evaporation

Answer 23

1. Boiling occurs at a definite temperature, that is, the boiling point. Evaporation occurs at any temperature of the liquid.
2. When boiling occurs, bubbles form. When there is evaporation, no bubbles form.
3. Boiling occurs throughout the liquid. Evaporation occurs at surface of liquid only.
4. Boiling requires heating. Evaporation does not require heating.

Question 24

Cooling effect of evaporation

Answer 24

• evaporation also requires latent heat of vaporisation. However, the latent heat is taken away from the liquid itself to cause the drop of temperature liquid.
• sweating causes the cooling of human body since the evaporation if sweat takes away latent heat of vaporisation from the body
• wet bodies may easily get cold in windy places

Question 25

Explanation of evaporation by molecular motion

Answer 25

• Molecules in a liquid possess a wide range of kinetic energies. Thus, some molecules have greater kinetic energy and move faster.
• When the fast moving molecules reach the surface, they have enough energy to overcome the attraction forces from the other molecules and escape into air.
• As the more energetic molecules escape, the average kinetic energy of the remaining molecules decreases, thus temperature of liquid drops.

Question 26

Factors affecting the rate of evaporation

Answer 26

1. Temperature of the liquid
   - The rate of evaporation increases with the temperature of the liquid.
2. Surface area of the liquid
   - The rate of evaporation increases with the surface area of the liquid
3. Movement of air above the liquid surface
   - The rate of evaporation would increase in windy places.
4. Humidity of air
   - The rate of evaporation would increase if their air is less humid.
   5.The boiling water of the liquid
   - rate of evaporation increases if the liquid has lower boiling point (more volatile)
   - Eg. alcohol has a lower boiling point than water, thud alcohol evaporates faster.