ABSTRACT
Water consists of molecules that are composed of
hydrogen and oxygen atoms. The molecules are in
constant motion, and there are attractive forces
between them. If a quantity of liquid water in a closed
container has a free surface that is in contact with air
say, some of the molecules have so much kinetic
energy that they can escape the attractive pull of
other water molecules. As a result, they escape into
the vapor region in which the intermolecular attractive
forces are negligibly small. A dynamic equilibrium is
established between the liquid water and its vapor
when the number of molecules leaving the liquid in a
given time equals the number that enters the liquid. At
this point of equilibrium, the pressure that the water
molecules exert on the walls of the container and the
liquid water is known as the saturation vapor pressure.
In the case of a plane or flat surface of water, this
pressure is a function only of temperature. One of the
most accurate expressions that relate the saturation
vapor pressure, ps (Pa), and temperature, T ( C), is
the Goff-Gratch[1] equation, namely
