What is Boyle's law?
Quick answer
Boyle’s law says that, at a constant temperature, the pressure of a fixed amount of gas is inversely proportional to its volume. Squeeze a gas into half the space and its pressure doubles. Written as a formula: P₁V₁ = P₂V₂. Irish scientist Robert Boyle published it in 1662.
Why it happens
A gas is made of molecules zooming around and hitting the walls of their container. Each collision pushes on the wall, creating pressure. If you squeeze the gas into a smaller space, the molecules hit the walls more often, so the pressure rises. Keeping the temperature constant means the molecules’ speed doesn’t change.
A quick example
A syringe holds 100 mL of air at 1 atmosphere of pressure. Push the plunger until the air takes up 50 mL:
P₂ = (1 atm × 100 mL) ÷ 50 mL = 2 atm
Boyle’s law in everyday life
| Situation | What happens |
|---|---|
| Syringe or bike pump | Pushing reduces volume and raises pressure, forcing air or liquid out |
| Scuba diving | Deeper water means higher pressure, so air in a diver’s lungs shrinks; divers must never hold their breath while rising |
| Ears on a plane | Air trapped in your middle ear expands as cabin pressure drops |
| Chip bags at altitude | Sealed bags puff up in the mountains where outside pressure is lower |
| Breathing | Your diaphragm enlarges your chest, lowering lung pressure so air rushes in |
Who was Robert Boyle?
Boyle (1627–1691) was an Anglo-Irish scientist and a founder of modern chemistry. With his assistant Robert Hooke, he built an air pump and did experiments with a J-shaped tube of mercury to measure trapped air. French physicist Edme Mariotte found the same law independently, so it’s sometimes called the Boyle–Mariotte law.
The other gas laws
| Law | Relationship | Holds constant |
|---|---|---|
| Boyle’s | Pressure ↑ when volume ↓ | Temperature |
| Charles’s | Volume ↑ when temperature ↑ | Pressure |
| Gay-Lussac’s | Pressure ↑ when temperature ↑ | Volume |
| Avogadro’s | Volume ↑ with more gas | Temperature and pressure |
They combine into the ideal gas law, PV = nRT.
Pressure also changes the temperature at which liquids boil; see what a boiling point is.
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