# What is Latent Heat?

Latent heat is the energy transferred during a change of state without changing the temperature.

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Up Learn – A Level physics (aqa)

## Thermal Physics

Back in Internal Energy, we looked at what happens when we add a heater to ice…

And we tracked its temperature over time on this graph.

For example, we saw that when the heater is turned on, the temperature of the ice initially increases

This is because the energy transferred by heating…

This is because the energy transferred by heating increases the average kinetic energy of the particles.

Then, at this point, the temperature stops increasing for a while…

And the ice turns into liquid water.

And so, here, the energy transferred by heating…

Here, the energy transferred by heating increases the average potential energy of the particles.

Now, we call the energy transferred during a change of state without changing the temperature, latent heat

Where latent means ‘hidden’, because when this energy is transferred, we don’t observe a temperature increase.

And so, if we continue to leave the heater on, and track what happens…

We can see that when energy is transferred to the material, the material changes from a solid, to a liquid, to a gas.

And conversely, when energy is transferred to the surroundings, the system changes from a gas, to a liquid, to a solid!

And this explains why we get worse burns from water vapour at 100 degrees Celsius than liquid water at 100 degrees Celsius.

When water vapour touches our skin, it turns back into liquid water.

So, as well as the energy transferred due to the temperature difference…

Energy is also transferred when the water changes from gas to liquid!

To sum up, latent heat is…
Latent heat is the energy transferred during a change of state without changing the temperature.