Gas Laws

Boyle's law states that for a fixed mass of dry gas at constant temperature, the volume is inversely proportional to the pressure. Push the molecules into half the space and they strike the walls twice as often, so the pressure doubles. As an equation, P × V = constant, which means P1V1 = P2V2 for two states of the same gas. The 'dry' part matters: water vapour would add its own pressure and spoil the result.

Charles's law states that for a fixed mass of dry gas at constant pressure, the volume is directly proportional to the absolute (kelvin) temperature. Heat the gas and its molecules move faster; to keep the pressure the same the gas must expand. The key trap is temperature: it must be in kelvin, not Celsius. V1/T1 = V2/T2 only works when T is measured from absolute zero.

If you cool a gas, its volume shrinks by 1/273 of its volume at 0°C for every degree Celsius. Extending this line, the volume would reach zero at -273°C. This lowest possible temperature, where molecular motion stops, is called absolute zero and is the start of the Kelvin scale. To convert, add 273 to the Celsius value: 27°C becomes 300 K. The gas laws are written in kelvin because only then is the relationship truly proportional.

Boyle's and Charles's laws can be merged into one statement: PV/T is constant for a fixed mass of gas. So P1V1/T1 = P2V2/T2 lets you handle a change where pressure, volume and temperature all alter at once. Standard temperature and pressure (STP) - 0°C (273 K) and 76 cm of mercury - gives a fixed reference for comparing gas volumes.