Physics · Kinetic Theory · NEET
Gay-Lussac's Law is at CONSTANT VOLUME. The gas is sealed in a rigid container that cannot expand, so V and the number of moles are fixed. Only pressure and temperature change together: P is proportional to T. Do not mix this up with Charles's Law, which keeps pressure constant and links volume with temperature.
Because the law says pressure is proportional to ABSOLUTE temperature. If you used Celsius, 0 degrees C would wrongly give zero pressure, and doubling from 10 C to 20 C would not double the pressure. Only the kelvin scale starts at true zero (absolute zero). Always convert: T(K) = T(C) + 273 (use 273.15 for exact work) before putting numbers into P1/T1 = P2/T2.
Both involve temperature, so students confuse them. Charles's Law: pressure fixed, volume is proportional to T (V/T = constant). Gay-Lussac's Law: volume fixed, pressure is proportional to T (P/T = constant). Trick to remember: Gay-Lussac has 'Pressure' — think 'Pressure cooker gets hot and the pressure rises' because it is a closed, fixed-volume pot.
From kinetic theory, heating raises the average kinetic energy of the molecules, so they move faster. In a rigid (fixed-volume) box the molecules cannot spread out, so they strike the walls harder and more often each second. More force per unit area on the walls means higher pressure. This is why an aerosol can may burst if thrown in fire.
On the kelvin (absolute) scale, yes: P versus T is a straight line through the origin, because P = (constant) times T. On the Celsius scale the line is still straight but it crosses the temperature axis at about -273 C (absolute zero) instead of at the origin. NEET usually asks for the kelvin graph, a straight line from the origin.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
P/T = constant, at fixed volume and fixed mass of gas. For two states this becomes P1/T1 = P2/T2, with T in kelvin.
It is named after the French chemist Joseph Louis Gay-Lussac (early 1800s). It is also called the pressure law or Amontons's law.
Yes. The ideal gas equation PV = nRT contains all the gas laws. If you hold V and n constant, then P is proportional to T, which is exactly Gay-Lussac's Law.
Use P1/T1 = P2/T2. So P2 = P1 x (T2/T1) = 1 x (450/300) = 1.5 atm. Pressure rises with temperature in the same ratio.
A pressure cooker or a sealed aerosol can: the volume is fixed, so heating raises the internal pressure. Car tyre pressure also rises on a hot day for the same reason.
