ABSTRACT
Gas is a substance that is in the gaseous state at ordinary
temperature and pressure whereas a vapor is the gaseous
state of any substance that is either a liquid or a solid in
ordinary conditions. For example, water at 25C and 1 atm pressure is a liquid and therefore, its gaseous form is a
vapor, and oxygen at same conditions is a gas. Water is in
the solid state as ice, in the liquid state as water, and in the
gaseous state as steam or water vapor. These physical
states exhibit different properties.[1]
Most substances can exist in one of these three states
and their physical properties are highly dependent on the
physical state.[2] In a gas, molecules are far apart (com-
pared to their diameters) making it very compressible, low
in density, assuming the volume and shape of its container,
and very free in molecular motion. In a liquid, the mole-
cules are held close together by one or more types of
attractive forces with very little empty space. This makes
liquid only slightly compressible, high in density, and free
in molecular motion whenmolecules slide past one another
freely and molecules do not break away from attractive
forces so liquid can flow and has definite volume but
assumes the shape of its container. In a solid, because
molecules are held rigidly in position with virtually no
freedom of motion (molecules vibrate about fixed posi-
tions). A number of solids are characterized by long-range
order, i.e., molecular arrangements are in regular config-
urations in three dimensions.[2] With very little intermole-
cular space, a solid is virtually incompressible, high in
density, and has a definite volume and shape. Water is a
rare exception, where the density of its solid state (ice) is
lower than the liquid state (4C) creating interesting natural events, such as ice floating in the ocean, creating
unique habitats for living animals both above and under
the water.
