P₁V₁ = P₂V₂ (at constant temperature)
Robert Boyle discovered in 1662 that for a fixed amount of gas at constant temperature, pressure and volume are inversely proportional: P₁V₁ = P₂V₂. Doubling the pressure halves the volume, and vice versa. This is because gas molecules in a smaller volume collide with the walls more frequently, creating higher pressure. Boyle's Law is one of the gas laws that combine to form the ideal gas law PV = nRT. It applies accurately at low pressures and high temperatures where gases behave ideally. At high pressures, real gas behavior deviates — molecules have finite volume and experience intermolecular attractions, described by the van der Waals equation. Practical applications: syringes, hydraulic systems, scuba diving (compressed air), and weather balloons expanding at altitude.