Which condition is required for Boyle's Law to hold?

• A. Constant Temperature
• B. Constant Volume
• C. Constant Pressure
• D. Constant Amount of Gas

Answer: (A)

The key idea is that pressure and volume are inversely related only when the temperature is kept constant (and the amount of gas stays the same). According to the ideal gas relation P V = n R T, if the temperature T and the amount n are constant, the product P V is a constant. That means as you compress the gas to a smaller volume, the pressure rises proportionally, and as you expand the volume, the pressure drops. If temperature were allowed to change, or the amount of gas changed, the product P V would no longer stay constant, and Boyle’s inverse relationship would not hold. The other conditions listed would not guarantee the inverse P–V relationship on their own.