Substances may exist in solid, liquid or gaseous form. These forms or phases differ from each other according to the random movement of their constituent atoms or molecules. In solids, molecules oscillate about a fixed point, whereas in liquids the molecules possess higher velocities and move more freely and thus do not bear a constant relationship in space to other molecules. The molecules of gases also move freely, but to an even greater extent.

Both gases and liquids are termed fluids. Liquids are incompressible and at constant temperature occupy a fixed volume, conforming to the shape of a container; gases have no fixed volume but expand to occupy the total space of a container.

Heating a liquid increases the kinetic energy of its molecules, permitting some to escape from the surface into the vapour phase. Random loss of molecules with higher kinetic energies from a liquid occurs in the process of vaporization. As these molecules possess higher kinetic states, this leads to a reduction in the energy state and cooling of the liquid.

Collision of randomly moving molecules in the gaseous phase with the walls of a container is responsible for the pressure exerted by a gas.

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