Physics · Kinetic Theory · NEET
Charles's Law is V/T = constant (volume divided by Kelvin temperature stays fixed) at constant pressure. PV = constant is Boyle's Law, which holds at constant temperature. Do not mix them: Charles's Law changes temperature, Boyle's Law changes pressure. If a NEET question fixes pressure and changes temperature, use V1/T1 = V2/T2.
Always use Kelvin. The direct proportion V is proportional to T only works with absolute temperature. Convert first: T(K) = t(Celsius) + 273. If you use Celsius, a temperature of 0 C would wrongly give zero volume, which is not what happens. Converting to Kelvin is the single most common step students forget in NEET numericals.
Charles's Law describes only how volume responds to temperature. If pressure also changes, both effects mix and V/T is no longer constant. Constant pressure isolates the temperature effect. When pressure also varies, you must use the full ideal gas equation PV = nRT (or the combined gas law P1V1/T1 = P2V2/T2) instead of Charles's Law alone.
If you extend the volume vs temperature graph back to where volume would become zero, all gases point to the same temperature: -273.15 C, which is 0 Kelvin, called absolute zero. Real gases liquefy before this, so it is a theoretical limit, but it is exactly why the Kelvin scale starts at -273.15 C. This straight-line extrapolation is a favourite NEET conceptual point.
On a V vs T(Kelvin) graph, Charles's Law is a straight line passing through the origin (0 K, 0 volume), showing direct proportion. On a V vs t(Celsius) graph it is still a straight line but it cuts the temperature axis at -273.15 C, not at the origin. Knowing which axis (Kelvin or Celsius) is used decides whether the line passes through the origin.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
At constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute temperature: V/T = constant.
V1/T1 = V2/T2, where T1 and T2 are absolute (Kelvin) temperatures and pressure is constant for a fixed mass of gas.
Convert: T1 = 27 + 273 = 300 K, T2 = 127 + 273 = 400 K. Charles's Law: V2 = V1 x (T2/T1) = 300 x (400/300) = 400 mL.
It applies well to real gases at low pressure and high temperature, where they behave nearly ideally. At high pressure or near liquefaction, real gases deviate from Charles's Law.
At constant pressure a gas expands by 1/273 of its volume at 0 C per degree Celsius rise, giving a coefficient of about 3.66 x 10 to the power -3 per K, matching Charles's Law.
