According to Kinetic Theory, the pressure exerted by a gas in a container is caused by the collisions of its molecules with the walls. Pressure changes directly with the number of molecules colliding per unit time.
When pressure on a given mass of gas is increased at constant temperature, the distance between molecules decreases and volume decreases. If volume is reduced to one half, molecules per unit volume double → collisions double → pressure doubles. This is Boyle’s law: volume of a given mass of gas is inversely proportional to its pressure at constant temperature.
Mathematically: V ∝ 1/P or P₁V₁ = P₂V₂ (at constant T and mass).
Robert Boyle used a J-shaped tube sealed at the short end. He poured mercury to trap air. Initially air at atmospheric pressure (760 mm Hg). Added mercury until height difference ~760 mm → pressure doubled (~2 atm). Volume of trapped air became half of initial. Mass & temperature constant. This verified P ∝ 1/V.
Figure 14.4 (described): trapped air, mercury columns, volume halved when pressure doubled.
Take a syringe, seal the nozzle. Push the plunger: volume decreases, pressure increases (feel resistance). Release: volume increases. Relate to Boyle’s law.
Differentiation: Provide formula triangle for P,V; ask advanced students to derive relationship from kinetic theory.