1787 – France

‘The volume of a given mass of gas at constant pressure is directly proportional to its absolute temperature’

In other words, if you double the temperature of a gas, you double its volume. In equation form: **V/T = constant**, or **V _{1}/T_{1} = V_{2}/T_{2}**, where

**V**is the volume of the gas at a temperature

_{1}**T**(in kelvin) and

_{1}**V**the new volume at a new temperature

_{2}**T**.

_{2}This principle is now known as Charles’ Law (although sometimes named after GAY-LUSSAC because of his popularisation of it fifteen years later – Gay Lussac’s experimental proof was more accurate than Charles’).

It completed the two ‘gas laws’.

A fixed amount of any gas expands equally at the same increments in temperature, as long as it is at constant pressure.

Likewise for a decline in temperature, all gases reduce in volume at a common rate, to the point at about minus 273degrees C, where they would theoretically converge to zero volume. It is for this reason that the kelvin temperature scale later fixed its zero degree value at this point.

CHARLES’ Law and BOYLE‘s Law may be expressed as a single equation, **pV/T = constant**. If we also include AVOGADRO‘s law, the relationship becomes **pV/nT = constant**, where **n** is the number of molecules or number of moles.

The constant in this equation is called the gas constant and is shown by **R**

The equation – known as the ideal gas equation – is usually written as **pV = nRT**

Strictly, it applies to ideal gases only. An ideal gas obeys all the assumptions of the kinetic theory of gases. There are no ideal gases in nature, but under certain conditions all real gases approach ideal behaviour.

###### Related sites

- NASA (nasa.gov)