Specific latent heat

The specific latent heat of a substance is a measure of how much heat energy is needed to melt or boil it. It is the energy needed to melt or boil 1 kg of the substance.

Different substances have different specific latent heats. The specific latent heat of a given substance is different for boiling than it is for melting. The table shows some examples:

SubstanceMelting (kJ/kg)Boiling (kJ/kg)
Water3342260
Lead23871
Oxygen13.9213

Investigating latent heat

The latent heat of water can be determined in the following way:

  1. an electrical heater is used to melt ice for several minutes
  2. the electrical energy used is calculated by multiplying the power of the heater by time
  3. the mass of water melted is recorded and the relationship between energy input and mass is calculated
  4. the amount of heat needed to change one kilogram of ice (the latent heat) is then determined

Here is the equation that relates energy to specific latent heat:

curriculum-key-fact
energy (J) = mass (kg) × specific latent heat (J/kg)

You will probably find your values close, but not identical, to those in the table above. Differences between your value and those in the table are likely to result from energy loss to the room you worked in.

The latent heat of steam is determined using a calorimeter and steam generator.

The latent heat of melting is sometimes called the latent heat of fusion, while the latent heat of boiling is sometimes called the latent heat of vaporisation.