The specific latent heat of a substance is the energy which is required to change 1 Kg of a substance from a certain physical condition to another physical condition without any change in temperature.
The unit for specific latent heat is JKg-1.
The graphs above shows how the temperature of a quantity of substance such as water changes over time when heat is supplied to it.
As you can see above,, all along the temperature from 0 to 273 K, water is in the form of solid, that is ice.
In this phase:
- When the temperature is raised, the water molecules vibrate even faster.
- Heat energy supplied is converted to kinetic energy.
All along the straight line at 273K, a change of phase from ice to water occurs.
- Even though heat is still supplied to it, the temperature does not increase all along.
- This is because the heat energy supplied is needed to separate the water molecules and not for the increasing their energy.
- The heat that is required in the change of phase from Solid to liquid is termed the latent heat of fusion.
At the end of the straight line at 273K, all of the solid (ice) has melted into liquid.
All along the graph from 273K to 373K, water only exist in the form of liquid only. Therefore, the temperature of water will increase when heat is supplied to it.
All along the graph of 373 K (the level phase), the change of phase from liquid to gas occurs.
Along the line:
- Water is boiling.
-it is observed that the temperature does not change even though heat is constantly supplied to the substance.
- Heat is required to separate the water molecules and to do the work of opposing air pressure when the liquid changes into gas.
-The heat required to convert liquid into gas is termed the latent heat of evaporation.
At the end of the level line at 373K, all of the liquid has been changed into gas.
At the graph from 373K to 473K, water is in the form of gas and the temperature rises when heat is supplied.
When there is cooling, the reverse process occurs.
Latent heat of fusion and latent heat of evaporation will be released.
Since the heat energy supplied during the change in phase cannot be detected by a thermometer, this type of heat is referred to as latent heat.
Therefore, the change of state is an 'energy change without any loss of temperature change' phenomenon.
Now I am going to discuss about the, Specific Latent heat of fusion. Specific latent heat of fusion, L of a substance is the quantity of heat which is required to change one unit mass of the substance from solid to liquid without any change of temperature at the melting limit.
Its unit is JKg-1.
Specific latent heat of fusion occurs at the melting point of the solid.
For example, 336000J of heat is required to change 1Kg of ice at 0°C.
Therefore the latent heat of fusion, L for ice is 336 000 JKg-1.
It has to be noted that when liquid solidifies, the specific latent heat of fusion will be released.
This condition occurs at the freezing limit of a liquid.
For example, when 1 Kg of water at 0°C solidifies to become 1 Kg of ice of 0°C, 336 000 J of heat are released.
If m Kg of solid or liquid is involved, the quantity, Q of heat absorbed or released is
Q = mL
where Q = quantity of heat that is absorbed or released
m = mass of substance
L = latent heat of fusion
Below are examples of substance with its specific latent heat
- Aluminum 3.96x10^5 JKg-1.
- Copper 2.05x10^5 JKg-1.
- Iron 2.67x10^5 JKg-1.
- Lead 0.23x10^5 JKg-1.
- Brass Unknown
- Magnesium 3.7x10^5 JKg-1.
- Zinc 1.1x10^5 JKg-1.
Hope you will understand what Specific latent heat is.