This entry contributed by Dana Romero

Portions of this entry contributed by David Rovnyak

In 1787, French physicist Jacques Charles observed that the volume of a gas under constant pressure increases or decreases with temperature. This behavior was quantified around 1808 by another French scientist, Joseph Gay-Lussac , who measured the thermal expansion of a gas as 1/267 of its original volume per degree Celsius. In 1847, Henri Regnault refined this value to 1/273, and also discovered that many gases violate this rule, which in principle holds only for so-called ideal gases.

Charles' law is represented by the formula

(1)

where V and are the final and initial volumes and is the change in temperature. More generally, it may be stated that for a gas at constant pressure,

(2)

where V is the volume, T is the temperature, and C is a constant.

When combined with the observation by Robert Boyle a century earlier that the volume of a gas at a constant temperature is inversely proportional to its pressure (Boyle's law), we arrive at

(3)

where T is expressed in degrees Kelvin rather than degrees Celsius and C is a constant. In combination with Avogadro's hypothesis, (3) takes the form

(4)

known as the ideal gas law, where R is the universal gas constant and n is the number of moles of the gas in the volume V.

Avogadro's Hypothesis, Boyle's Law, Ideal Gas Law, Universal Gas Constant