Latent heat

Not all heat exchange in nature stops changing body temperatures. In some cases there is change of physical state of these bodies. In this case we call the calculated amount of heat latent heat.

The amount of latent heat (Q) is equal to the product of body mass (m) and a proportionality constant (L).

Like this:

The proportionality constant is called the phase shift latent heat and refers to the amount of heat that 1 g of the calculated substance needs to change from one phase to another.

In addition to depending on the nature of the substance, this numerical value depends on each change in physical state.

For example, for water:

Latent heat of fusion 80cal / g
Latent heat of vaporization 540cal / g
Latent heat of solidification -80cal / g
Latent heat of condensation -540cal / g


Q> 0: The body melts or vaporizes.

Q <0: The body solidifies or condenses.


How much heat does a liter of water need to vaporize? Data: water density = 1g / cm³ and latent heat of water vaporization = 540 cal / g.

Like this:

Heating curve

When studying the latent heat values, we observed that they do not depend on the temperature variation. Thus we can make a temperature graph as a function of the amount of heat absorbed. We call this chart Heating curve: