Specific Latent Heat

The specific latent heat of a substance is the energy absorbed or released per unit mass of the substance when it undergoes a certain change of state. If an amount of a substance, with mass $m$, absorbs or releases an amount of energy (latent heat) $E$ when it undergoes a certain change of state, the specific latent heat, $L$, is given by

$$L = \frac{E}{m}$$

The specific latent heat of a substance has different values depending on which two states the substance changes between. The specific latent heat of fusion is the specific latent heat of a substance for when it changes between solid and liquid. (Here, ‘fusion’ has nothing to do with nuclear fusion.) The specific latent heat of vaporisation is the specific latent heat of a substance for when it changes between liquid and gas.

An Example

$40\,\mathrm{g}$ of a substance changes from a liquid to a solid, releasing $2000\,\mathrm{J}$ of energy. What is the specific latent heat of fusion of the substance?

We can use the formula

$$L = \frac{E}{m}$$

$$L = \frac{2000\,\mathrm{J}}{40\,\mathrm{g}}$$

$$L = 50\,\mathrm{\frac{J}{g}}$$