Physics · Kinetic Theory · NEET
Boyle's Law is at CONSTANT temperature (isothermal) with a fixed mass of gas. Only pressure and volume change. If temperature is held constant instead of pressure, you get Boyle's Law (P inversely proportional to V). Do not confuse it with Charles's Law, where pressure is constant and V is proportional to T.
From P V = constant we get P = constant / V. Plotting P against V gives a hyperbola (a curved line called an isotherm) because P falls off as 1/V. But if you plot P against 1/V, then P = constant times (1/V), which is the equation of a straight line through the origin. Its slope equals the constant, which is nRT for that fixed mass and temperature.
Two things are held fixed: the mass (amount) of gas and the temperature. Because of this, the product P V stays constant. So if pressure goes up, volume must go down by the same factor to keep P V unchanged.
Boyle's Law is exact only for an ideal gas. NCERT shows that real gases follow it well only at LOW pressure and HIGH temperature, where molecules are far apart and behave nearly ideally. At high pressure and low temperature, real gases deviate because molecular size and attractions matter.
Take the ideal gas equation PV = nRT. For a fixed mass, n (number of moles) is constant. At constant temperature, T is constant, and R is always constant. So the whole right side nRT is a fixed number, which means PV = constant. That is exactly Boyle's Law.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
At constant temperature, the pressure of a fixed mass of gas is inversely proportional to its volume, so P V = constant.
P V = constant, or between two states P1 V1 = P2 V2, at constant temperature and fixed mass.
An isotherm is the P vs V curve for a gas at a single fixed temperature. Each isotherm is a rectangular hyperbola, and a higher-temperature isotherm lies farther from the origin.
Pressure doubles. Since P V is constant, halving V forces P to become twice as large to keep the product the same.
Robert Boyle discovered it in 1661, as noted in the NCERT Kinetic Theory chapter.
