Gaseous state is a state of matter in which the substance does not have any specific shape or volume. It adopts the form and size of its container. The fundamental macroscopic properties of gases are pressure, volume, temperature and mass of the gas. The relationships among pressure, temperature and volume of a gas lead to Gas laws. According to Boyle’s Law, the pressure (P) of a given mass of gas is inversely proportional to its volume (V), provided that the temperature of the gas remains constant. i.e, the volume of gas increases as the pressure decreases.
This video explains how to study the variation in volume with pressure for a sample of air at constant temperature by plotting graphs between P and V, and between P and 1/V.