Under Boyle's Law, if the volume of a gas is halved at constant temperature, what happens to the pressure?

• A. It remains the same.
• B. It doubles.
• C. It halves.
• D. It becomes quadruple.

Answer: (B)

When a gas keeps the same amount of substance and constant temperature, its pressure and volume are inversely related: P × V = constant. So if the volume is halved, the pressure must adjust to keep the product PV the same. Mathematically, P2 = P1 × (V1/V2) = P1 × (V1/(V1/2)) = 2 × P1. In other words, the pressure doubles. This makes sense physically: squeezing the same number of gas molecules into half the space increases the frequency of collisions with the container walls, raising the pressure. The other options would violate PV = constant unless the volume change were different (e.g., needing a quarter of the volume to quadruple the pressure).