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energy to a solid may lead to its fusion and eventually to its evaporation.
To melt a solid (Fig. 11.3) it is necessary to supply a given quantity of
energy to each unit mass. The heat of melting (fusion) of a substance is the
heat necessary to convert a unit mass of solid into liquid at the same

temperature and pressure. During the process of crystallization the same
quantity of the heat energy, absorbed during melting, is discharged. The
temperature of melting coincides with the temperature of crystallization.

The heat energy of melting is equal to

Q  Lm , (5.2.1)

where m is a mass of melted body, L is the specific heat of melting.
The temperature at which the liquid changes into the solid state is its
freezing point. For crystalline substances the freezing temperature and

melting temperature coincide and are sharply defined. Substances that are
not crystalline, such as wax or glass, gradually soften and do not have
definite melting points.

The molecules of a liquid move with a wide range of instantaneous
velocities. When the liquid is heated, this range and the average speed
increase. Some of the molecules near the surface attain sufficient kinetic

energy to escape the forces of attraction that confine the less energetic
molecules to the liquid. When the space above the surface of the liquid is
enclosed some vapor molecules return to the surface of the liquid and are
captured. As more molecules escape, the number returning to the surface

also increases. When the number of the molecules returning to the surface
is equal to the number escaping, the space above the liquid is said to be
saturated, and above the liquid we have the saturated vapor. The saturated

vapor pressure of any given liquid only depends on the temperature. Even
at relatively low temperature the most energetic molecules evaporate from
the surface of a liquid.
Evaporation is the escape of molecules from the surface of a liquid. As

the temperature of a liquid is raised, the evaporation proceeds more
rapidly. Eventually bubbles of vapor form in the liquid and rise to the
surface. The temperature, at which the vapor bubbles form in the volume

of the liquid and rise to the surface, is the boiling point or the boiling
temperature .
The distinction between evaporation and boiling should now be clear.

Both involve the change of phase from liquid to gas, which is called
vaporization. Evaporation is vaporization from the surface alone whereas
boiling is vaporization within the volume of the liquid. Since the boiling

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