What remains constant in Boyle's Law?

• A. Pressure
• B. Volume
• C. P times V
• D. Temperature

Answer: (C)

Pressure and volume move in inverse harmony for a fixed amount of gas at a constant temperature, so their product stays unchanged. This is why the quantity that remains constant is the product of pressure and volume (PV is constant when n and T are fixed). If you compress the gas and shrink its volume, the pressure rises just enough to keep PV the same; if you expand the volume, the pressure drops accordingly. The other possibilities don’t stay fixed: pressure alone or volume alone change as you adjust the other variable, and temperature is held constant as a condition, not the invariant quantity that remains constant alongside P and V.