Dalton's Law of Gas

Dalton’s law states that in a mixture of different gases, such as air, the sum of the partial pressures of all the gases equals the total pressure. The partial pressure of a gas is the pressure that gas would exert if it was the only gas present. This law, expressed mathematically, is:

where each subscripted P value is the partial pressure of a different gas.

Air, for example, consists mostly of nitrogen and oxygen, with small quantities of argon, water vapor, and carbon dioxide. The partial pressure of nitrogen accounts for 78 percent of the total pressure exerted by Earth’s atmosphere; oxygen accounts for 21 percent; and argon accounts for 0.9 percent. Together, these three gases account for 99.9 percent of air pressure. Water vapor, carbon dioxide, and all the other trace gases present in the atmosphere combined contribute only a tenth of a percent.

Charles’s Law of Gas

French chemist Jacques Charles discovered that the volume of a gas at constant pressure changes with temperature. As the temperature of the gas increases, so does its volume, and as its temperature decreases, so does its volume. French chemist Joseph Guy-Lussac refined Charles’s theory, and the law is also known as Guy-Lussac’s law.

Raising the temperature of a gas causes the gas to fill a greater volume as long as pressure remains constant. Gases expand at a constant rate as temperature increases, and the rate of expansion is similar for all gases. Charles's law (also called Gay-Lussac’s law) describes the relationship between volume and temperature of an enclosed gas. The law says that at constant pressure, the volume of a fixed number of particles of gas is directly proportional to the absolute (Kelvin) temperature, mathematically expressed as:

V = k T

where T is temperature on the Kelvin scale (see Temperature: Temperature Scales). If the temperature of a given amount of gas is doubled, for example, its volume will also double (as long as pressure remains unchanged).

Boyle’s Law of Gas

Boyle’s law, developed by English scientist Robert Boyle, states that:

The smaller the volume a given amount of gas is squeezed into, the greater the pressure the gas exerts on the walls of its container. Boyle's law, a mathematical equation that more precisely describes this relationship, states that at constant temperature, the volume of a given quantity of gas varies inversely with the pressure exerted on it. Mathematically, this relationship can be expressed:

V = k (1/P)

where V is volume; k is Boltzmann’s constant (equal to 1.38 × 10-23 and named for the Austrian physicist Ludwig Boltzmann); and P is pressure. If the pressure on a given amount of gas is doubled, for example, its volume will decrease by one-half (as long as the temperature of the gas remains unchanged). Conversely, if the pressure is decreased by one-half, the volume will double.