Gas laws

Gas particles move fast and hit the walls of their container, causing pressure. Squeezing, heating or adding gas changes the pressure in predictable ways.

Key facts

  • Kelvin = °C + 273. Absolute zero is 0 K (−273°C).
  • Boyle’s law (constant temperature): pV = constant.
  • Constant volume: p/T = constant. Constant pressure: V/T = constant (T in kelvin).
  • Ideal gas: pV = nRT.

Worked examples

Example 1

A gas at 100 kPa in 6 L is squeezed to 2 L at constant temperature. Find the new pressure.

  1. p₁V₁ = p₂V₂
  2. 100 × 6 = p₂ × 2 p₂ = 300 kPa

Answer: 300 kPa

Example 2

A tyre at 27°C has pressure 200 kPa. It heats to 57°C. Find the new pressure (volume constant).

  1. 27°C = 300 K, 57°C = 330 K.
  2. p₂ = 200 × 330 ÷ 300 = 220 kPa

Answer: 220 kPa