Specific Latent Heat
- A certain amount of energy is required to change the state of a certain mass of a substance
- This amount of energy is known as the latent heat
- The specific latent heat is defined as:
The amount of energy required to change the state of 1 kg of a substance with no change in temperature
- There are two types of specific latent heat:
- Specific latent heat of fusion
- Changing the state between a solid and liquid
- Solid to liquid, or liquid to solid
- Changing the state between a solid and liquid
- Specific latent heat of vaporisation
- Changing the state between a liquid and gas
- Liquid to gas, or gas to liquid
- Latent heat is represented by the symbol L with units joules per kilogram (J/kg)
The changes of state with heat supplied against temperature
Latent Heat of Fusion
- The specific latent heat of fusion is defined as:
The energy required to convert 1 kg of a substance between a solid and a liquid state with no change in temperature
- This applies when melting a solid or freezing a liquid
- When a solid substance is melted, its temperature stays constant until all of the substance has melted
- The latent heat of fusion is the amount of energy needed per kg for all the particles in the substance to overcome the intermolecular forces of attraction holding them together in their solid state
- If a substance in its liquid state is frozen, the substance will solidify at the same temperature as its melting point
- In this case, the latent heat of fusion is the amount of energy per kg transferred away from the substance until all the particles in the substance have succumbed to the intermolecular forces of attraction that hold them together in their solid structure
Latent Heat of Vaporisation
- The specific latent heat of vaporisation is defined as:
The energy required to convert 1 kg between a liquid and a gaseous state with no change in temperature
- This applies when vaporising a liquid or condensing a gas
- When a liquid substance is vaporised, its temperature will stay constant until all of the substance has vaporised
- The latent heat of vaporisation is the amount of energy per kg needed for all the particles in the substance to overcome the intermolecular forces of attraction holding them together in their liquid state
- If a substance is a gas and is condensed, it will condense at the same temperature as its boiling point
- In this case, the latent heat of vaporisation is the amount of energy per kg transferred away from the substance until all the particles in the substance have succumbed to the intermolecular forces of attraction that hold them together in their liquid state
Exam Tip
The specific latent heat of fusion and vaporisation value of all substances will be provided for you in the exam question, so you do not need to memorise the value of any.
However, make sure to include 'with no change in temperature' in your definition of specific latent heat to be awarded full marks.
Use these reminders to help you remember which type of latent heat is being referred to:
- Latent heat of fusion = imagine ‘fusing’ the liquid molecules together to become a solid
- Latent heat of vaporisation = “water vapour” is steam, so imagine vaporising the liquid molecules into a gas
But remember that the change of state can go in either direction!
In my experience of teaching this topic to GCSE students, the graphs and the word ‘latent’ can cause some confusion. Latent just means hidden. So the energy being transferred into the system (by heating it) causes the temperature to rise to the melting of a substance, we can see this happening using a thermometer. But when the substance reaches its melting point, we can continue to transfer energy to the system but we don’t see the temperature rising any more, it stays at the melting point. So what happens to this energy? What is it being used for? It appears to be hidden or latent because we can’t see its effects.
Once the substance has fully melted, the temperature begins to rise again. And when it reaches the boiling point, the same thing happens. The temperature stays constant even though energy is still being transferred into the system. What do these two events have in common? The period of latent energy transfers happened as the substance was changing state. Therefore, the energy transferred to the system at these points must be used for the state change. The energy is transferred to the molecules or particles and they use it to overcome the intermolecular forces of attraction holding them in their solid or liquid state. Physicists call this latent heat.
