MELTING AND BOILING

SEC 3 PHYSICS

On the Molecular Level

• We have learnt previously that when molecules absorb heat energy, they will start to move faster.

• When the molecules have reached a certain speed, the energy that they absorb will now be used to break the attractive forces between them.

On a Molecular Level (cont’d)

• When the energy absorbed is used to break the attractive forces between them, there is no increase in speed.

• Which means energy absorbed can only be used for either one of the purpose (break attractive forces OR increase in speed). Only one change is taking place at any time.

So what do we see?

• When the energy is used to increase the speed of the molecule, we see a rise in temperature of the substance.

• When the energy is used to break attractive forces between molecules, we see the substance changing in state.This energy which brings about a change in state is known as LATENT HEAT

Changes in States

melting

Boiling

CONDENSATION

FREEZING

Solid

Gas

Liquid

Gain in heat

Loss in heat

MELTING AND SOLIDIFICATION

• When a solid changes to a liquid on heating, we call it meltingIn this instance, heat is ABSORBED by the substance

• When a liquid changes to a solid on cooling, we call it solidification (freezing)In this instance, heat is LOST by the substance

MELTING AND SOLIDIFICATION

• In both cases, the heat that is absorbed or lost is known as the latent heat of fusion.

• When this latent heat is absorbed or lost, there is no change in temperature.

• This means when a substance is undergoing a change in state, the temperature will remain constant.

The Temperature-time Graph

temperature

time

This part of the graph shows a constant temperature which means this is when the substance is changing state

Effect of Impurities on Freezing Point

• When impurities are added to water, it would lower the freezing point of water. This means that water now freezes at a lower temperature.

Effect of Impurities on Freezing Point

Example: I have 2 cups of water, A and B.I add salt to B. When I try to freeze the water in both cups, A would freeze at 0oC but B would freeze maybe at –10oC.This means B would stay as a liquid even at 0oC or lower until the temperature reaches –10oC.When A becomes solid ice at 0oC, B is still liquid.

Effect of Impurities on Freezing Point

The reverse situation would be true. (When melting.)

A (no salt added) B (salt added)

Ice cube A would melt at 0oC.

But B would melt(turn into liquid) at a temperature even lower.

Effect of Pressure on Melting Point• Melting point of ice is lowered by an

increase in pressure• At higher pressure, an ice cube would

melt at temperatures lower than 0oC.• At lower pressure (on the mountain), ice

would only melt at temperatures higher than 0oC. Which is why there can be snow on the mountain top even when surrounding temperatures are above 0oC.

• Example of pressing two ice cubes together.• When pressed together, the pressure they

experience is higher, so the melting point is lowered causing the ice to melt.

• When hand releases, the pressure they experience is lowered, so the water will now freeze at higher temperature.

• Another example is walking on ice.• And ice skating.

Effect of Pressure on Melting Point

boiling AND condensation

• When a liquid changes to a gas on heating, we call it boilingIn this instance, heat is ABSORBED by the substance

• When a gas changes to a liquid on cooling, we call it condensation.In this instance, heat is LOST by the substance

Boiling AND condensation

• In both cases, the heat that is absorbed or lost is known as the latent heat of vaporisation.

• When this latent heat is absorbed or lost, there is no change in temperature.

• This means when a substance is undergoing a change in state, the temperature will remain constant.

Effect of Impurities on Boiling Point

• Adding impurities will raise the boiling point of an object.

• That means to say the liquid will now boil at a higher temperature.

Effect of Pressure On Boiling Point• Lower pressure would lower the boiling

point of water.• Water will boil very quickly on the

mountain top but the temperature reached is lower than 100oC.

• Increasing pressure would raise the boiling point of water.

• Water will boil at a higher temperature above 100oC.

Specific Latent HeatSpecific Latent Heat

• Recall that the energy absorbed or released causing a change in state is known as latent heat.

• Example: latent heat of fusion latent heat of vaporisation

Specific Latent HeatSpecific Latent Heat

• This is the heat absorbed or released when 1 kg of the substance undergoes a change in state.

• There are 2 types of specific latent heat:

Specific latent heat of fusionSpecific latent heat of vaporisation

Specific Latent Heat of Specific Latent Heat of FusionFusion

• This is the heat required by 1 kg of a substance to change from solid to liquid or vice versa without a change in temperature

• Formula: Lf = mlf

• Unit: Jkg-1

Specific Latent Heat of Specific Latent Heat of VaporisationVaporisation

• This is the heat required by 1 kg of a substance to change from liquid to gas or vice versa without a change in temperature

• Formula: Lv = mlv

• Unit: Jkg-1

Example:4400 J of heat was supplied to 2kg of water at 100oC. All of the water turned into steam. What is the specific latent heat of vaporisation of water?

Solution:

Lv = mlv

4400 = 2 (lv)

lv= 4400 / 2

lv=2200 Jkg-1

Evaporation• Liquid changing into gas.• Seems similar to boiling but there

are differences.

DifferencesDifferences

BoilingBoiling EvaporationEvaporation•Occurs at fixed temperature

•Occurs at any temperature

•Quick process •Slow process

•Takes places within liquid•Takes places only on surface

•Bubbles are formed •No bubbles are seen

•Temperature remains constant

•Temperature may change

•Heat supply by energy source

•Heat supplied by surroundings

Factors Affecting rate of Evaporation

• TemperatureTemperature of liquid and surroundings.