Cool wax!Can you stick the sheet in please?

Sketch a predicted graph

Time (mins)

Temp (°C)

waterwax

?

Today’s lesson

• Describe melting and boiling in terms of energy input without a change of temperature

• Distinguish between boiling and evaporation• Describe condensation and solidification• State the meaning of melting point and boiling

point• Use the terms latent heat of vaporisation and

latent heat of fusion and give a molecular interpretation of latent heat

Solids, liquids and gases

Cooling!

Latent heat

When pure substances cool, the temperature stops changing as the substance changes from a liquid to a solid.

Temp (°C)

Time (mins)

Melting point

Latent heat

When the molecules of a substance settle into the regular pattern of a solid, energy is released as bonds are formed. This energy released is called latent heat. This stops the temperature from falling.

(“latent” = “hidden”)

Latent heat

The opposite happens when a solid makes. Heat is needed to break the bonds between the solid particles (increasing their potential energy instead of raising the temperature (kinetic energy))

Temp (°C)

Time (mins)

Melting point

solid

liquid

The same happens when boiling

Specific Latent heat

The specific latent heat of a substance tells us how much energy is needed to change the state of 1 kg of substance at constant temperature.

Solid to liquid/liquid to solid

or

liquid to gas/gas to liquid

Another formula!

Energy = mass x specific latent heat

E = mL

Specific Latent Heat

The specific latent heat of fusion (melting) of ice at 0 ºC, for example, is 334000 J/kg. This means that to convert 1 kg of ice at 0 ºC to 1 kg of water at 0 ºC,

334000 J of heat must be absorbed by the ice.

1 kg 1 kg

334000 J absorbed

All at 0°C

Specific Latent Heat

Conversely, when 1 kg of water at 0 ºC freezes to give 1 kg of ice at 0 ºC, 334000 J of heat will be released to the surroundings.

334000 J released

1 kg 1 kgAll at 0°C

Specific Latent Heat of Vaporisation

For water at its normal boiling point of 100 ºC, the latent specific latent heat of vaporization is 2260000 J/kg. This means that to convert 1 kg of water at 100 ºC to 1 kg of steam at 100 ºC, 2260000 J of heat

must be absorbed by the water.

All at 100°C

2260000 J input

1 kg 1 kg

Latent heat

Conversely, when 1 kg of steam at 100 ºC condenses to give 1 kg of water at 100 ºC, 2260 kJ of heat will be released to the surroundings.

All at 100°C

2260000 J released

1 kg 1 kg

Example

• The specific latent heat of fusion (melting) of ice is 334 000 J/kg. How much energy is needed to melt 5kg of ice at 0°C to 5 kg of water at 0°C?

• Energy = mL = 5 x 334 000 = 1670000 J

IB level calculation

Calculate the amount of heat required to completely convert 50 g of ice at 0 ºC to steam at 100 ºC. The specific heat capacity of water is 4.18 kJ/kg/°C. The specific latent heat of fusion of ice is 334 kJ/kg, and the specific heat of vaporization of water is 2260 kJ/kg.

50g50g

0°C 100°C

An example calculation

Heat is taken up in three stages:

1. The melting of the ice.

2. The heating of the water.

3. The vaporization of the water.

0°C

0°C

0°C 100°C

100°C 100°C

Stage 1

1. Heat taken up for converting iceat 0ºC to water at 0ºC

mass of water x latent heat of fusion= 0.050 (kg) x 334 (kJ.kg-1) = 16.7 kJ

0°C

0°C

Stage 2

2. Heat taken up heating the waterfrom 0 ºC to the boiling point, 100 ºC

mass of water x specific heat capacity x temperature change= 0.05 (kg) x 4.18 (kJ.kg-1.°C-1)x 100 (ºC)= 20.9 kJ

0°C 100°C

Stage 3

3. Heat taken up vaporising thewater

mass of water x latent heat of vaporization0.05 (kg) x 2260 kJ.kg-1 = 113 kJ

100°C 100°C

The answer

The sum of these is

 

16.7 + 20.9 + 113 = 150.6 kJ (151 kJ)

Got it?

Just time for a quick dog accident

50 kg

Ooops!

50 kg

Let’s try some questions

Boiling and evaporation

Evaporation

Consider a beaker of water at room temperature

Evaporation

The molecules of water are moving around at different speeds, some fast, some slow.

speed of molecule (m/s)

# of molecules at a particular speed

Average speed

Evaporation

If a molecule is at the surface, and moving fast enough, it may escape the liquid. This is called evaporation.

Freedom!

EvaporationSince the average speed of the remaining molecules must now be lower, the temperature of the liquid drops (since temperature is a measure of the kinetic energy of the molecules). To gain the additional energy necessary to break free, the individual molecule must steal heat energy from neighbors.

Freedom!

Evaporation

Evaporation can thus take place at any temperature.

Boiling?

Boiling

Boiling occurs when vapour is produced in the body of the liquid.

What’s in the bubbles?

Boiling

Boiling occurs when vapour is produced in the body of the liquid.

The bubble contains only water vapour, not air!

Boiling

Boiling occurs when vapour is produced in the body of the liquid. This only happens at the boiling point of the liquid.

The bubble contains only water vapour, not air!

To summarize:

Evaporation takes place only at the surface of the liquid and can take place at any temperature.

To summarize:

Boiling occurs when vapour is produced in the body of the liquid. This only happens at the boiling point of the liquid.

Boiling means

bubbles!

OK, now try to answer some questions!

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