1. 1.3 Change of State 1.3.1 Change of State
1.3.2 Latent Heat and Specific Latent Heat
1.3.3 Evaporation

2. Prior Knowledge Linkage
Heat flow
Transfer processes
Internal energy
Internal kinetic energy
Internal potential energy
Molecular motion
Molecular separation
Temperature
State of matters

3. 1.3.1 Change of State Change of state Fusion Solidification
Vaporization
Condensation
Boiling
Evaporation
Melting point
Boiling point
Any temperature

4. SOLID LIQUID GAS Changes of states
Matter around us mainly exists in three states (物態): solid (固體), liquid (液體) and gas (氣體).
Heating
SOLID
LIQUID
GAS
Cooling
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5. The melting point and freezing point
A substance changes from solid to liquid at a temperature called the ; the same substance changes from liquid to solid at the constant temperature called the
Factors affecting values of melting point
Higher pressure
Melting point becomes
Easier to but more difficult to
More impurities
A thread acts like a knife to cut an ice cube.
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6. Factors affecting values of boiling point
The boiling point
A substance may change from liquid to gas in the process of at a temperature called the ; the same substance may also change from gas to liquid at the constant temperature.
Factors affecting values of boiling point
Higher pressure
Boiling point becomes
More difficult to but easier to
More impurities
Do you know why water takes longer time to boil on mountains?
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7. 1.3.2 Latent Heat and Specific Latent Heat
Fusion
Solidification
Vaporization
Condensation
Latent heat of fusion
Latent heat of vaporization
Specific latent heat of fusion
Specific latent heat of vaporization

8. Cooling curve Typical cooling curve Cooling curve Temperature / oC
Liquid octadecan-1-ol
thermometer
Hot Water
Typical cooling curve
Temperature falls at a rate here because energy is at a rate here (slope is ).
Temperature / oC
Temperature remains constant here. Is energy releasing?
Time / min
Experiments
Cooling curve
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9. Latent heat
Latent heat (潛熱) is the energy absorbed or released by a substance during its changes of state a change of temperature.
Unit of L:
Latent heat of fusion is the energy absorbed by a substance during its changes from to state a change of temperature.
Latent heat of vaporization is the energy absorbed by a substance during its changes from to state (except in ) a change of temperature.
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10. Microscopic interpretation of latent heat
Extension
Microscopic interpretation of latent heat
The internal potential energy of an object arises from the forces between molecules. During change of state upon heating, molecules gain more to overcome the attractive force, and have more freedom to move around. The internal potential energy while the internal kinetic energy , so that the temperature also .
gains k.e. (and p.e.)
gains k.e. (and p.e.)
gains k.e. (no p.e.)
gains p.e.
gains p.e.
liquid
gas
solid
Gain total internal energy (k.e.+p.e.)
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11. Specific latent heat
The specific latent heat (比潛熱) of a substance is the amount of energy required to change the state of of the substance. or
l : Specific heat capacity
E : Energy required
m : mass
Unit of l :
The specific latent heat of fusion of a substance is the amount of energy required to change of the substance form solid to liquid.
The specific latent heat of vaporization of a substance is the amount of energy required to change of the substance form liquid to gas.
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12. Specific latent heat of fusion of water
Experiments
Specific latent heat of fusion of water
Control
set - up
Experimental set - up
Ice
Joulemeter
12 V a.c. power supply
Mass of water in beaker of control set-up = kg
Mass of water in beaker of experiment set-up = kg
Initial joulemeter reading = J
Final joulemeter reading = J
Standard value
= 3.34  105 J kg-1
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13. Precautions of experiment
Reasons
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14. Possible sources of errors of experiment
Effects on obtained value
Ice cubes taken out from refrigerators are usually sticky to wet hands. Do you know why?
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15. Specific latent heat of vaporization of water
Experiments
Specific latent heat of vaporization of water
rotating disc
kilowatt-hour meter
Electronic balance
Why is a kW h meter needed in this experiment?
kW h meter calibration = turns / kW h
Number of rotations counted = 40
Mass of water boiled away = 0.05 kg
Standard value
= 2.26  106 J kg-1
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16. Precautions of experiment
Reasons
Should a lid be used in the experiment to reduce heat loss to the surroundings? Why?
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17. Possible sources of errors of experiment
Effects on obtained value
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18. Example
How much energy is required to change 0.2 kg of ice at 0 oC to steam at 100 oC.
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19. Example
Energy is transferred at an equal rate to equal masses of substance X and Y. Their temperature-time graphs are shown below.
Temperature / oC
Time / min
Substance X
Substance Y
Which substance has a higher
(i) melting point? (ii) boiling point?
specific latent heat of (i) fusion (ii) vaporization?
specific heat capacity in solid states?
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20. Example What would happen when 1 kg of ice meets 1 kg of steam?
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21. Latent heat of vaporization
1.3.3 Evaporation
Vaporization
Boiling
Evaporation
Temperature
liquid
Bubbles
Latent heat of vaporization

22. Occurrence of evaporation below boiling point
Do you notice that wet plants get dry eventually after raining?
Evaporation (蒸發) is the process in which a liquid change into gas (vapour) the boiling point.
Boiling
Evaporation
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23. Cooling effect of evaporation
When you leave water after swimming or bathing, you feel cold. Do you know why?
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24. Cooling by evaporation in refrigerators
Electric pump to pump the around the circuit.
Evaporating coils refrigerator
Condensing coils refrigerator
Energy removed from foods as freon
Energy given out to surrounging as freon .
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25. Condensation – the opposite of evaporation
Condensation is the process in which substance changes from to state when is
When a container of ice-cream is removed from the freezer, a mist forms around it. Do you know why?
The temperature of the ice-cream is much than room temperature. When the ice-cream or ice on it melts, large amount of is absorbed from the surrounding air. The temperature of the air quickly and so water vapour in the air quickly to form a mist.
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26. Warming effect of condensation
Evaporation on the surface of a body can the body down. On the other hand, condensation on the surface of a body can the body up.
The use of steam in cooking can make food being cooked more because the steam a large amount of energy to the food during condensation.
Other examples:
Preparing coffee by steam
Steam bath
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27. Microscopic interpretation of evaporation
Extension
Microscopic interpretation of evaporation
Molecules in a liquid move at speeds and collide with each other. Molecules at the surface can escape from the liquid if they gain enough energy from these collisions. The escaped molecules become molecules of vapour.
Molecules gain enough energy to escape
Energy taken away from body
As molecules carrying energy have escaped, the average kinetic energy of the liquid The temperature of the liquid therefore
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28. Factors affecting rate of evaporation
How can wet clothes dry quickly?
Extension
Evaporation is faster for
because
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