Question

Assertion (A): At constant temperature, when the volume of an ideal gas is doubled the pressure of the gas reduces to half. Reason (R): According to Boyle's law, pressure is inversely proportional to volume at constant temperature.

Properties of gases are easier to understand than those of solids and liquids. This is mainly because in a gas, molecules are far from each other and their mutual interactions are negligible except when two molecules collide. The perfect gas equation can be written as PV = µRT where µ is the number of moles and R = NA kB is a universal constant. The temperature T is absolute temperature. Choosing kelvin scale for absolute temperature, R = 8.314 J mol-1K-1. If we fix µ and T in the equation, we get PV = constant i.e., keeping temperature constant, pressure of a given mass of gas varies inversely with volume. This is the famous Boyle's law. Next, if you fix P, shows that V ∝ T i.e., for a fixed pressure, the volume of a gas is proportional to its absolute temperature T (Charles' law).