Phase change
Latent heat.
Energy must be supplied to a substance to melt it or to boil it - in other words, to make
it change state. This energy does not increase the substances temperature, and for this
reason it is known as latent heat (the word latent means “hidden”).
The energy needed to change a liquid into a gas is called the latent heat of
vaporization. The energy needed to change a solid into a liquid is called the latent heat
of fusion. (Here, the word “fusion” means “melting”) To compare different substances
fairly, we measure the energy required to change the state of 1 kg of the substances.
So specific latent heat is defined as follows:
• The specific latent heat of vaporization (Lv) is the energy required to cause 1 kg
of a substance to change state from liquid to gas at its boiling point.
• The specific latent heat of fusion (Lf.) is the energy required to cause 1 kg of a
substance to change state from solid to liquid at its melting point.
This energy is needed to break the bonds between particles.
Measuring latent heat.
To determine the specific latent heat of a substance, 1 kg of the substance must be
heated at its melting or boiling point until it entirely changes state. The amount of
energy supplied must be measured. Then we have:
Where:
Q – thermal energy, J
L – specific latent heat, J/kg
m – mass, kg