92 The Poetry of Physics and The Physics of Poetry
As with solids the molecules of liquids are in thermal motion. As the
temperature of the liquid is increased, this thermal motion increases until
the boiling point is reached (which for water is 100° Centigrade). The
thermal motion is then so great that the remaining molecular bonds of the
liquid phase are broken and the substance enters the gas phase in which
there are no bonds between the individual molecules.
Even before the boiling point is reached, however, a certain amount
of evaporation of a liquid into a gas takes place. This phenomenon of
evaporation is responsible for the creation of clouds over large water
masses and hence subsequent precipitation that follows. Evaporation
can be explained easily with our molecular model of liquids. At the
temperatures of the liquid phase the thermal motion of the molecules is
quite violent. Occasionally a molecule near the surface has enough
kinetic energy to break its ties with the other molecules and it escapes
from the liquid. It then enters the atmosphere as water vapor. At any
given temperature the vapor pressure that is the amount of water vapor in
the air is fixed. An equilibrium between the number of water molecules
escaping the liquid and the number accidentally falling back and being
reabsorbed is established.
In the gas phase all of the molecular bonds have been broken. Except
for collisions the molecules of a gas are completely independent of each
other. Most of the volume occupied by the gas is empty space. A volume
of water vapor is about 1700 times that of an equivalent weight of water
in the liquid phase.
A gas is characterized by three quantities; its volume, pressure and
temperature. The volume of the gas is defined as the volume of the
container holding the gas. The pressure is the force per unit area exerted
on the sides of the container by the molecules bouncing off the sides of
the container. Conservation of momentum demands that each time a
molecule bounces off the wall, it transfers an amount of momentum
equal to twice its momentum in the direction perpendicular to the wall.
The temperature of the gas is a measure of the motion of the individual
molecules. It is in fact, as we will show below, directly proportional to
the kinetic energy of the molecules.
The original work on gases by Robert Boyle revealed that for a fixed
temperature the pressure and the volume are inversely proportional or
putting it in an equivalent form, the product of the pressure times the
volume is a constant at a fixed temperature. This result is consistent with
the atomic picture of a gas since decreasing (increasing) the volume of