Which statement expresses Boyle's Law?

• A. PV = nRT.
• B. P is proportional to V at constant T.
• C. V is proportional to T.
• D. P is inversely proportional to V at constant T.

Answer: (D)

The main idea here is how pressure relates to volume when the amount of gas and the temperature are fixed. Boyle's law says pressure is inversely proportional to volume for a fixed amount of gas at constant temperature, so the product PV stays constant. That means if you increase the volume, the pressure drops proportionally, and if you decrease the volume, the pressure rises in the inverse way. A practical way to remember this is to imagine a piston: isothermal compression (decreasing V) increases P so that PV remains the same.

The other expressions mix in temperature or use a direct proportionality that doesn’t match Boyle’s scenario. PV = nRT is the general ideal gas law, not the isothermal P–V relation. V ∝ T at constant P is Charles’ law, relating volume and temperature rather than P and V under fixed T. P ∝ V would imply pressure rises with volume, which contradicts Boyle’s behavior when temperature is held constant. Therefore, the statement that P is inversely proportional to V at constant temperature best expresses Boyle's law.