Boyle's Law

When the volume of a gas is decreased (for example by a piston), the pressure increases in the same ratio, if the temperature does not change. Compressing the volume to a half of its size doubles the pressure.

p1 \cdot V1 = p2 \cdot V2

also well known as:

p \cdot V = \textup {constant}

When a vacuum is produced and the pressure is decreased by a large amount, the gases expand to very large volumes. This occurs also with contamination on the inner walls of the vacuum chamber, which evaporates (for example fingerprints).

with equal numbers of molecules (temperature kept constant)

for the same number of molecules (constant temperature)