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

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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