Boyle's Law

Boyle's Law describes what happens to a fixed amount of gas when temperature stays the same: if the gas is compressed into a smaller space, its pressure rises, and if it is allowed to expand, its pressure falls. This happens because the same gas particles are moving around with the same average kinetic energy, so changing the volume changes how often they hit the container walls. Fewer collisions per unit area means lower pressure; more collisions means higher pressure.

Use this law for gas problems where the temperature and amount of gas are constant, such as syringes, pistons, balloons in a sealed container, or compressed-air systems. It is especially useful for finding a new pressure or volume after a gas is squeezed or expanded.

• Using this law when temperature changes, even though Boyle's Law only applies at constant temperature
• Mixing up pressure and volume so that both are treated as increasing or both as decreasing
• Forgetting to keep units consistent, such as comparing litres with cubic centimetres without converting