If the volume of a confined gas is doubled while the temperature remains constant, what happens to the pressure?

• A. Becomes one-half of its original value
• B. Doubles
• C. Remains the same
• D. Increases by a factor of three

Answer: (A)

Pressure is inversely related to volume when temperature is held constant. This means doubling the volume, while keeping the amount of gas the same, reduces the frequency of molecular collisions with the container walls, so the pressure drops. Mathematically, P1V1 = P2V2; if V2 is twice V1, then P2 is half of P1. So the pressure becomes one-half of its original value. The other possibilities—pressure staying the same, increasing, or increasing by a larger factor—would violate this inverse relationship under constant temperature.