Cambridge IGCSE Chemistry · Syllabus 0620 · States of Matter
Volume of a gas
What is Volume of a gas?
The space occupied by a gas, which is always the space available inside its container rather than a fixed property of the gas itself. Two things change it. Raising the temperature of a gas at constant pressure increases its volume, and lowering the temperature decreases it, provided the container can change size. Increasing the pressure on a gas at constant temperature decreases its volume, and reducing the pressure increases it. If the gas is sealed in a rigid container whose walls cannot move, the volume cannot change at all, and heating the gas raises its pressure instead. In every case the number of particles and the size of each particle stay the same; only the space between them changes.
This definition is part of the States of Matter chapter in Cambridge IGCSE Chemistry.