1.3 Change of State
1.3.1 Change of State
1.3.2 Latent Heat and Specific Latent Heat
1.3.3 Evaporation
Prior Knowledge Linkage
Transfer processes
Heat flow
Internal kinetic energy Internal potential energy
Temperature State of matters
Molecular motion Molecular separation
Internal energy
Solidification Vaporization
1.3.1 Change of State
Change of state
Fusion Condensation
Melting point Boiling point
Boiling Evaporation
Any temperature
Changes of states
Concept maps
Matter around us mainly exists in three states ( 物態 ): solid (固體 ), liquid (液體 ) and gas (氣體 ).
Heating
Cooling
Melting point becomes .
The melting point and freezing point
Concept maps
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
Easier to but more difficult to .
A thread acts like a knife to cut an ice cube.
Boiling point becomes .
The boiling point
Concept maps
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
Do you know why water takes longer time to boil on mountains?
More difficult to but easier
to .
1.3.2 Latent Heat and Specific Latent Heat
Latent heat of fusion Latent heat of vaporization
Solidification VaporizationFusion Condensation
Specific latent heat of fusion Specific latent heat of vaporization
Temperature falls at a rate here because energy is at a rate here (slope is ).
Temperature / oC
Time / min
Cooling curve
Concept mapsExperiments Cooling curve
Typical cooling curve
Liquid octadecan-1-ol
thermometerHot Water
Liquid octadecan-1-ol
Temperature remains constant here. Is energy releasing?
Latent heat
Concept maps
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.
Microscopic interpretation of latent heat
Concept maps
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 .
solid liquid gas
gains p.e.
gains k.e. (and p.e.)
gains k.e. (and p.e.)
gains k.e. (no p.e.)
gains p.e.
Gain total internal energy (k.e.+p.e.)
The specific latent heat of vaporization of a substance is the amount of energy required to change of the substance form liquid to gas.
Specific latent heat
Concept maps
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 capacityE : Energy requiredm : 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.
Specific latent heat of fusion of water
Concept maps
Experiments Specific latent heat of fusion of water
Control set - up
Experimental set - up
Ice
IceJoulemeter
12 V a.c. power supply
Mass of water in beaker of control set-up = 0.012 kgMass of water in beaker of experiment set-up = 0.048 kgInitial joulemeter reading = 24 500 JFinal joulemeter reading = 38 800 J
Standard value = 3.34 105 J kg-1
Precautions Reasons
Precautions of experiment
Concept maps
Ice cubes taken out from refrigerators are usually sticky to wet hands. Do you know why?
Possible sources of errors Effects on obtained value
Possible sources of errors of experiment
Concept maps
Specific latent heat of vaporization of water
Concept maps
Experiments Specific latent heat of vaporization of water
kW h meter calibration = 1 2 00 turns / kW hNumber of rotations counted = 40Mass of water boiled away = 0.05 kg
Standard value = 2.26 106 J kg-1
Why is a kW h meter needed in this experiment?
rotating disckilowatt-hour
meter
Electronic balance
Precautions Reasons
Precautions of experiment
Concept maps
Should a lid be used in the experiment to reduce heat loss to the surroundings? Why?
Possible sources of errors Effects on obtained value
Possible sources of errors of experiment
Concept maps
Example
Concept maps
How much energy is required to change 0.2 kg of ice at 0 oC to steam at 100 oC.
Example
Concept maps
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 XSubstance Y
Which substance has a higher(a) (i) melting point? (ii) boiling point?(b) specific latent heat of (i) fusion (ii) vaporization?(c) specific heat capacity in solid states?
Example
Concept maps
What would happen when 1 kg of ice meets 1 kg of steam?
1.3.3 Evaporation
Vaporization
Boiling Evaporation
Temperature
liquid
Latent heat of vaporization
Bubbles
Occurrence of evaporation below boiling point
Concept maps
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
Cooling effect of evaporation
When you leave water after swimming or bathing, you feel cold. Do you know why?
Concept maps
Top Related