# Question #362b8

Apr 22, 2014

Latent heat is the energy (see note below) released or absorbed during a phase change, where the temperature does not change. This is because all of the heat is used up in the phase change, rather than into changing temperature.

For a more in-depth explanation specifically of latent heat of fusion, see the link

http://socratic.org/questions/what-does-latent-heat-of-fusion-mean

For a more in-depth explanation specifically of latent heat of vaporization, see the link

Or watch the video shown below:

Note:
Latent heat is actually the total amount of enthalpy (a kind of energy) necessary to accomplish a phase change.

Phase changes are generally considered at constant pressure, rather than constant volume. Because a kg of say, 100°C steam, occupies a much greater volume than a kg of 100°C water, a lot of work has to be done to push the environment out of the way as that water expands to become steam. (The fact that vaporization does a lot of work is why we use steam to power a large number of the turbines in the world.)

Enthalpy, $H$, is a quantity that includes the energy $U$ that goes into the substance in the new phase, plus the work, $W = P \setminus \Delta V$, to expand or contract the substance into its new phase at the same pressure:

$H = U + P \setminus \Delta V$,

where $P$ is the ambient pressure, $\setminus \Delta V$ is the change in volume, and $U$ is the internal energy of the substance in its new phase. Latent heat of vaporization is typically much bigger than latent heat of fusion because of the much larger change in volume involved.