# Using Specific Latent Heat to Calculate Energy Required for a Change of State

How to work out an energy change using specific latent heat of fusion or specific latent heat of vaporization..

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Up Learn – A Level physics (aqa)

## Thermal Physics

We’ve seen that the energy required to change 1 kilogram of material from the solid state to liquid state is known as the specific latent heat of fusion…

And the energy required to change 1 kilogram of material from the liquid state to gas state is known as the specific latent heat of vaporisation.

And these values depend on the material itself.

Now, if we want to change the state of more than 1 kilogram of material…

If we want to change the state of more than 1 kilogram of material, we need more energy…

And if we want to change the state of less than 1 kilogram of a substance, we need less energy.

Therefore, the amount of energy required for a change of state depends on two things:

The mass of the material

And the specific latent heat of fusion or vaporisation.

And we can combine these factors in this formula [Q = ml]

Where Q stands for the energy required to change the state of a substance without a change in temperature, in joules…

m stands for the mass of the substance, in kilograms

And l stands for the specific latent heat of fusion or vaporisation, in joules per kilogram.

For example, here we have 20 kilograms of ice…

And ice has this specific latent heat of fusion.

[20.0 kg and 3.34 x 105 J kg-1]

To work out how much energy is required to turn all the ice into liquid water, we substitute these values into this formula…[Q = ml] [20.0 x 3.34 x 105 ]

And calculate to get this!  [6.68 x 106 J]

So now, what is the value for Q in this example?

To work out the value for Q, we substitute the variables into this formula…

Which calculates to give this!

Finally, you can find this formula in the data and formulae sheet!

To sum up, to calculate the energy required for a change of state, we use this formula [Q = ml]

Where…

Q is the energy required to change the state of a substance without a change in temperature, reported in joules

M is the mass of the substance, reported in kilograms

And l is the specific latent heat of fusion or vaporisation, reported in joules per kilogram