Which gas law is described by P1V1 = P2V2 when temperature is constant?

• A. Boyle's Law.
• B. Charles' Law.
• C. Avogadro's Law.
• D. Gay-Lussac's Law.

Answer: (A)

When temperature is held constant, the pressure and volume of a fixed amount of gas are inversely related. This means their product stays the same: P1V1 = P2V2. Boyle's law expresses this isothermal behavior: if you compress the gas (decrease V), the pressure rises proportionally, and if you expand (increase V), the pressure falls, so that P × V remains constant. This idea also follows from the ideal gas law PV = nRT, and with n and T fixed, PV must be constant, yielding the same relation between the initial and final states.

For context, the other gas laws describe different conditions: Charles' Law links volume to temperature at constant pressure, Avogadro's Law links volume to the amount of gas at constant temperature and pressure, and Gay-Lussac's Law links pressure to temperature at constant volume. Under the given condition of constant temperature and fixed amount of gas, the isothermal P–V relationship is the one that gives P1V1 = P2V2.