The gas laws
The gas laws are a set of relationships that describe how a gas responds to changes in pressure, volume, temperature and the amount of substance present. They emerged from experiments in the seventeenth, eighteenth and nineteenth centuries and remain a standard part of introductory chemistry and physics. Because gases consist of widely separated particles in rapid, random motion, their bulk behaviour can be summarised by a few simple rules that hold well under ordinary conditions.
Boyle's law states that, at constant temperature, the volume of a fixed quantity of gas is inversely proportional to its pressure: squeezing a gas into a smaller space raises its pressure. Charles's law states that, at constant pressure, the volume of a gas is directly proportional to its absolute temperature, so heating a gas makes it expand. Gay-Lussac's law links pressure and temperature at constant volume. These individual relationships combine into the ideal gas law, usually written as PV = nRT, which ties together pressure, volume, amount and absolute temperature through the gas constant R.
The kinetic theory of gases explains these laws in terms of particle motion: pressure arises from countless collisions of particles with the container walls, and raising the temperature increases the average speed of those particles. Animations and simulations are well suited to this subject because they can show the particles moving, speeding up when heated, and striking the walls more often or more forcefully as conditions change.
Frequently asked questions
- What does Boyle's law say?
- At a constant temperature, the volume of a fixed amount of gas is inversely proportional to its pressure, so reducing the volume increases the pressure.
- What is the ideal gas law?
- It is the combined relationship PV = nRT, connecting pressure, volume, the amount of gas and absolute temperature through the gas constant.
- Why must temperature be measured on an absolute scale?
- The gas laws relate volume and pressure to absolute temperature in kelvin, where zero corresponds to the theoretical absence of particle motion, keeping the proportions correct.
