According to Boyle's Law, the pressure of a gas is inversely proportional to its volume when temperature and amount remain constant. Therefore, if a fixed sample of gas increases in volume, the pressure of the gas must decrease assuming temperature and amount of the gas remain constant. In other words, if the volume increases the pressure decreases; conversely, if the volume decreases the pressure increases proportionally.

Mathematically, Boyle's Law can be expressed as:

P₁V₁ = P₂V₂

Where,

P₁ = Initial pressure of the gas

V₁ = Initial volume of the gas

P₂ = Final pressure of the gas

V₂ = Final volume of the gas