$$l = \dfrac{L}{m}$$
where $l$ is the specific latent heat of the substance (in J kg-1),
$L$ is the heat transfer or energy absorbed, also known as latent heat (in J), and
$m$ is the mass of the substance (in kg)
Specific latent heat is an intrinsic property of the material and is independent of the amount of substance. Different substances have different specific latent heats, which reflect how much energy is required to change their state.
The following is a table of some of the commonly known specific latent heats.
| Substance | Specific Latent Heat of Fusion (J/kg) | Specific Latent Heat of Vaporisation (J/kg) | Melting Point (°C) | Boiling Point (°C) |
|---|---|---|---|---|
| Water (Hâ‚‚O) | 334,000 | 2,260,000 | 0 | 100 |
| Ethanol (Câ‚‚Hâ‚…OH) | 104,000 | 846,000 | -114 | 78 |
| Mercury (Hg) | 11,400 | 296,000 | -39 | 357 |
| Iron (Fe) | 247,000 | 6,300,000 | 1,538 | 2,862 |
| Gold (Au) | 64,500 | 1,590,000 | 1,064 | 2,807 |
| Oxygen (Oâ‚‚) | 13,900 | 213,000 | -218 | -183 |
| Nitrogen (Nâ‚‚) | 25,700 | 201,000 | -210 | -196 |
You will notice that the specific latent heat of vaporisation is typically much higher than the specific latent heat of fusion. This is because the process of vaporisation requires breaking more intermolecular bonds compared to fusion.
During fusion (melting), a substance transitions from a solid to a liquid. In this process, the molecules need to overcome the forces that hold them in a fixed position in the solid state, but they are not completely separated from each other. The molecules in the liquid state are still close together, though they can move around more freely.
During vaporisation, the substance transitions from a liquid to a gas. This process requires breaking almost all the intermolecular forces that hold the molecules together in the liquid state. In the gaseous state, the molecules are far apart and move independently, which requires significantly more energy.