Changes of state

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 Gain in heat Loss in heat Solid Gas Liquid

MELTING AND SOLIDIFICATION
When a solid changes to a liquid on heating, we call it melting
In 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 0 o C but B would freeze maybe at –10 o C.
This means B would stay as a liquid even at 0 o C or lower until the temperature reaches –10 o C.
When A becomes solid ice at 0 o C, 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 0 o C. 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 0 o C.
At lower pressure (on the mountain), ice would only melt at temperatures higher than 0 o C. Which is why there can be snow on the mountain top even when surrounding temperatures are above 0 o C.

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 boiling
In 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 100 o C.
Increasing pressure would raise the boiling point of water.
Water will boil at a higher temperature above 100 o C.

Specific 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 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 fusion
Specific latent heat of vaporisation

Specific Latent Heat of Fusion
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: L f = m l f
Unit: Jkg -1

Specific Latent Heat of Vaporisation
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: L v = m l v
Unit: Jkg -1

Example:
4400 J of heat was supplied to 2kg of water at 100 o C. All of the water turned into steam. What is the specific latent heat of vaporisation of water?
Solution: L v = m l v 4400 = 2 ( l v ) l v = 4400 / 2 l v =2200 Jkg -1

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

Differences
Heat supplied by surroundings
Heat supply by energy source
Temperature may change
Temperature remains constant
No bubbles are seen
Bubbles are formed
Takes places only on surface
Takes places within liquid
Slow process
Quick process
Occurs at any temperature
Occurs at fixed temperature
Evaporation Boiling

Factors Affecting rate of Evaporation
Temperature
Temperature of liquid and surroundings.

http://www.slideshare.net	58
http://tangrox.pbworks.com	10
http://fisicablog22.blogspot.com	1
http://tools.discoveryeducation.com	1

Changes of state

by meenng on Apr 16, 2007

Accessibility

Categories

Tags

Upload Details

Usage Rights

4 Embeds 70

Statistics

Changes of state — Presentation Transcript