Q = mL
m
Note the absence of a ∆T in the expression Q = mL
m . There is no ∆T in the expressio n because
there is no temperature change in the process. The whole phase change takes place at one
temperature.
So far, we have talked about the case of a solid sample, at the melting temperature, which is in
contact with a hotter object. Heat flows into the sample, melting it. Now consider a sample of
the same substance in liquid form at the same temperature but in contact with a colder object. In
this case, heat will flow from the sample to the colder object. This heat loss from the sample
does not result in a decrease in the temperature. Rather, it results in a phase change of the
substance of which the sample consists, from liquid to solid. This phase change is called
freezing. It also goes by the name of solidification. The temperature at which freezing takes
place is called the freezing temperature, but it is important to remember that the freezing
temperature has the same value as the melting temperature. The heat-per-mass that must flow
out of the substance to freeze it (assuming the substance to be at the freezing temperature
already) is called the latent heat of fusion, or L f . The latent heat of fusion for a given substance
has the same value as the latent heat of melting for that substance:
L f = L m
The amount of heat that must flow out of a sample of mass m in order to convert the entire
sample from liquid to solid is given by:
Q = mL
f
Again, there is no temperature change.
The other two phase changes we need to consider are vaporization and condensation.
Vaporization is also known as boiling. It is the phase change in which liquid turns into gas. It
too (as in the case of freezing and melting), occurs at a single temperature, but for a given
substance, the boiling temperature is higher than the freezing temperature. The heat-per-mass
that must flow into a liquid to convert it to gas is called the latent heat of vaporization L v . The
heat that must flow into mass m of a liquid that is already at its boiling temperature (a.k.a. its
vaporization temperature) to convert it entirely into gas is given by:
Q = mL
v
Condensation is the phase change in which gas turns into liquid. In order for condensation to
occur, the gas must be at the condensation temperature, the same temperature as the boiling
temperature (a.k.a. the vaporization temperature). Furthermore, heat must flow out of the gas, as
it does when the gas is in contact with a colder object. Condensation takes place at a fixed
temperature known as the condensation temperature. (The melting temperature, the freezing
temperature, the boiling temperature, and the condensation temperature are also referred to as the
melting point, the freezing point, the boiling point, and the condensation point, respectively.)
The heat-per-mass that must be extracted from a particular kind of gas that is already at the
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