# Specific Latent Heat: Fusion and Vaporisation

Specific latent heat is latent heat per unit mass. Specific latent heat of fusion relates to a solid becoming a liquid, and specific latent heat of vaporisation relates to a liquid becoming a gas.

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Up Learn – A Level physics (aqa)

## Thermal Physics

We’ve previously seen that latent heat is the energy transferred during a change of state without a change in temperature.

Now, the amount of energy transferred during a change of state depends on the material.

For example, it takes this many joules of energy to melt 1 kilogram of ice…

And it takes this many joules of energy to melt 1 kilogram of lead.

Now, another word for melting is fusion.

This might sound a bit counterintuitive, since we may normally think of a liquid fusing together to form a solid…

But fusion actually comes from the latin word ‘fundere’, which means ‘to melt’.

And so, we call the energy required to change 1 kilogram of material from the solid state to the liquid state without a change of temperature, the specific latent heat of fusion.

We add ‘specific’ before it because it’s defined for a particular unit mass of material

In this case, 1 kilogram!

Next, it takes this many joules of energy to turn liquid water into a gas…

And it takes this many joules of energy to turn liquid lead into a gas.

Now, the transition of a liquid into a gas is known as vaporisation.

And so, we call the energy required to change 1 kilogram of a material from the liquid state to the gas state without a change of temperature, the specific latent heat of vaporisation.

Finally, both the specific latent heat of fusion and the specific latent heat of vaporisation are the amount of energy required for a change of state per kilogram

The units for both are joules per kilogram.

So, for example, the specific latent heat of fusion of lead is this…

And the specific latent heat of vaporisation of water is this!

To sum up…

The energy required to change 1 kilogram of material from the solid state to the liquid state without a change of temperature is known as the specific latent heat of fusion

And the energy required to change 1 kilogram of a material from the liquid state to the gas state without a change in temperature is known as the specific latent heat of vaporisation

And the units for both are joules per kilogram.