was first formulated as an experimental gas law which described how the pressure of a gas decreased when the volume of said gas increased.

A more formal description of states that the pressure exerted by a mass of ideal gas is inversely proportional to the volume it occupies **if** temperature and amount of gas remain unchanged.

Mathematically, this can be written as

##P## ##alpha 1/V##, or ##PV = “constant”##

This is where a ##k## is usually seen, as it is often used to describe a constant value. So the ##k## you are referring to is

##PV = “constant” = k##

This can be easily derived from the , ##PV = nRT##, for the conditions specified by Boyle’s law.

We need to keep the amount of gas, which represents the number of , and the temperature constant. Since ##R## is a constant already, the becomes

##PV = nRT = k##

Therefore, ##k## must be constant in order to allow for a relationship to be set between pressure and volume.