Which statement describes Boyle's Law?

• A. At a constant volume, the pressure and temperature of a gas are directly related.
• B. At a constant temperature, the volume and the temperature of a gas are inversely related.
• C. At a constant volume, the volume and pressure of a gas are inversely related.
• D. At a constant temperature, the volume and pressure of a gas are inversely related.

Answer: (D)

Boyle's Law shows how pressure and volume of a gas are linked when the temperature stays the same. The key idea is that pressure and volume are inversely related: as volume goes down, pressure goes up, and as volume goes up, pressure goes down, while the amount of gas and the temperature remain constant. Mathematically, P × V is constant for a fixed amount of gas at a constant temperature. Think of a piston: if you squeeze it and keep temperature steady, the gas gets more pressurized as the space shrinks, and if you let it expand, the pressure drops.

The other statements mix up which variables are held constant or which quantities relate. For example, pressure and temperature at constant volume is a different gas law, and volume and temperature at constant pressure follow Charles’s law. Saying volume and pressure are inversely related at a constant volume isn’t meaningful, because the volume isn’t changing in that case.