Definition · Plain-language
Boyle’s law
Boyle’s law states that, for a fixed amount of gas at constant temperature, pressure and volume are inversely proportional — squeeze a gas into half the space and its pressure doubles.
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Pressure and volume are inversely related
Boyle’s law, named after Robert Boyle who described it in 1662, applies to a fixed amount of gas kept at a constant temperature. Under those conditions, the pressure and volume of the gas are inversely proportional: if you reduce the volume, the pressure rises in exact proportion, and if you increase the volume, the pressure falls. Multiplying pressure by volume always gives the same number, expressed as PV = constant, or P₁V₁ = P₂V₂ when comparing two states. Squeeze a gas into half its volume and its pressure doubles; let it expand to twice the volume and the pressure halves.
Why it happens
The kinetic theory of gases explains Boyle’s law through the motion of molecules. A gas exerts pressure because its countless molecules constantly collide with the walls of their container. Squeeze the same number of molecules into a smaller volume and they strike the walls more often, so the pressure rises. Because the temperature is held constant, the molecules’ average speed is unchanged — only the frequency of collisions increases. This molecular picture is why the law applies to ideal gases and is a very good approximation for real gases at ordinary pressures and temperatures.
Everyday examples
Boyle’s law is at work whenever a gas is compressed or expanded at steady temperature. Pushing the plunger of a sealed syringe compresses the trapped air, raising its pressure until you feel the resistance. A scuba diver’s lungs face higher pressure at depth, so a held breath would expand dangerously on a fast ascent as the surrounding pressure drops — a key safety rule in diving. The law also describes how a bicycle pump builds pressure, and how the bubbles in a fizzy drink grow as they rise to lower-pressure water near the surface.
Key facts
At a glance
- Definition: at constant temperature, gas pressure is inversely proportional to volume
- Equation: PV = constant, or P₁V₁ = P₂V₂
- Condition: fixed amount of gas, constant temperature
- Effect: halving the volume doubles the pressure
- Why: smaller volume means more frequent molecular collisions with the walls
- Part of: the gas laws, alongside Charles’s law and others
Common misconceptions
What people often get wrong
Often heard: Boyle’s law holds even when the gas is heated or cooled.
Actually: It applies only at constant temperature. If the temperature changes, you must use the combined gas law, because temperature also affects pressure and volume.
Often heard: Doubling the volume doubles the pressure.
Actually: Pressure and volume are inversely related, so doubling the volume halves the pressure. Their product, not their sum, stays constant.
Often heard: Compressing a gas adds more gas molecules and that is why pressure rises.
Actually: The amount of gas is fixed. Pressure rises because the same molecules hit the smaller container’s walls more frequently, not because more gas is added.
Going deeper
