At constant temperature, the pressure of a gas is inversely proportional to its volume. This is known as which gas law?

• A. Amontons' Law
• B. Charles' Law
• C. Boyle's Law
• D. Graham's Law

Answer: (C)

When you keep the amount of gas and its temperature fixed, the pressure adjusts inversely with volume so that their product remains constant. This is Boyle's law. In the ideal gas equation PV = nRT, if n and T are constant, P = nRT / V, which explicitly shows P is inversely proportional to V. So compressing the gas (reducing V) raises the pressure, while expanding it lowers the pressure, keeping PV constant.

The other relations describe different situations: a constant volume with increasing temperature raises pressure (Amontons’ law), a constant pressure with increasing temperature enlarges volume (Charles’ law), and Graham’s law deals with diffusion/effusion rates depending on molar masses—not a P–V relationship.