If the volume of a fixed amount of gas at constant temperature is doubled, what happens to pressure?

• A. Pressure doubles
• B. Pressure halves
• C. Pressure remains the same
• D. Pressure quadruples

Answer: (D)

For a fixed amount of gas at constant temperature, pressure and volume are inversely related (Boyle’s law): P ∝ 1/V, with P1V1 = P2V2. If the volume doubles, then V2 = 2V1, so P2 = P1V1 / (2V1) = P1/2. In other words, the pressure becomes half as large because the gas molecules have more space to move and collide with the container walls less frequently. The idea that pressure would quadruple would require the volume to shrink to one-quarter of its original size, not double.