All pure substances display similar behavior in the gas phase. At 0° C and 1 atmosphere of pressure, one mole of every gas occupies about 22.4 liters of volume. Molar volumes of solids and liquids, on the other hand, vary greatly from one substance to another. In a gas at 1 atmosphere, the molecules are approximately 10 diameters apart. Unlike liquids or solids, gases occupy their containers uniformly and completely. Because molecules in a gas are far apart, it is easier to compress a gas than it is to compress a liquid. In general, doubling the pressure of a gas reduces its volume to about half of its previous value. Doubling the mass of gas in a closed container doubles its pressure. Increasing the temperature of a gas enclosed in a container increases its pressure.

Because different gases act similarly, it is possible to write a single equation relating volume, pressure, temperature, and quantity of gas. This Ideal Gas Law and the related Boyle's Law, Law of Charles and Gay-Lussac, and Dalton's Law are central to understanding the more complex behavior of real gases.

Ideal Gas Law:

PV = nRT

Boyle's Law:

PV = k_{1}

Law of Charles and Gay-Lussac:

V = k_{2}T

Dalton's Law:

P_{tot} = P_{a} + P_{b}

where:

P is pressure, P_{tot} is total pressure, P_{a} and P_{b} are component pressures

V is volume

n is number of moles

T is temperature

k_{1} and k_{2} are constants