In Boyle's law, which quantity is kept constant?

Boyle's law was stated by the Irish-born scientist Robert Boyle in 1662. It states that at a constant temperature, the volume of a fixed mass (fixed amount) of a gas is inversely proportional to its pressure. In other words, if the pressure on a gas is doubled, its volume is halved, provided the temperature and the amount of gas do not change.

Mathematically: P ∝ 1/V, or PV = constant, or P₁V₁ = P₂V₂ (at constant temperature and fixed amount of gas).

A graph of pressure against volume at constant temperature is a curve called an isotherm (a rectangular hyperbola).

Examples of Boyle's law:

• When you squeeze a balloon, its volume decreases and the pressure inside increases.
• As a weather balloon rises, the outside air pressure falls, so the balloon expands.
• Bubbles of air released by a diver grow larger as they rise to the surface, where the pressure is lower.
• A syringe draws in liquid when the plunger is pulled back, because the volume increases and the pressure inside falls.

Other gas laws:

• Charles's law (Jacques Charles, 1787): at constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute temperature (V ∝ T).
• Gay-Lussac's law: at constant volume, the pressure of a fixed mass of gas is directly proportional to its absolute temperature (P ∝ T).
• Avogadro's law: at constant temperature and pressure, equal volumes of all gases contain equal numbers of molecules (V ∝ n).

Combining these laws gives the ideal gas equation: PV = nRT, where R is the universal gas constant (about 8.314 J per mol per K). Real gases follow these laws closely at low pressure and high temperature.

Why not the other options?

• ✗Pressure is kept constant in Charles's law.
• ✗Volume is kept constant in Gay-Lussac's law.
• ✗Good to know
• ✗The amount of gas is fixed in Boyle's law, but the defining condition is constant temperature.