ideal gas

A gas is countless atoms or molecules zooming around inside a container, with mostly empty space between them. An ideal gas is a clean model in which we pretend those molecules are tiny points that do not attract each other and interact only by bouncing (perfectly elastic collisions). Everyday image: like a swarm of minute bouncy specks racing around a room, so small compared with the room that they almost never touch one another.

More precisely, an ideal gas is a model gas built on these idealizing assumptions: (1) the molecules take up negligible volume compared with the container; (2) there are no forces between them except during the brief moments of collision; (3) collisions are perfectly elastic, so no kinetic energy is lost; and (4) they move randomly and obey Newton's laws. Under these assumptions the pressure, volume, temperature, and amount of gas are tied together by the ideal gas law, P V = n R T.

Why it matters and an honest caveat: real gases behave almost exactly like an ideal gas when they are dilute (low pressure) and warm (well above their boiling point), because then the molecules are far apart and moving fast. The model breaks down at high pressure or low temperature, where molecular size and attractions start to matter (see the van der Waals equation). It is an idealization, and we should say so, but a very useful and surprisingly accurate one for everyday air.