4.1.2—Ideal gases have zero particle volume and no

An ideal gas is a model whose particles move rapidly and randomly, occupy negligible volume, exert no intermolecular attraction or repulsion, and collide elastically. Its temperature is linked to the particles' average kinetic energy.

Because the particles are treated as point-like and non-attracting, the model predicts gas behaviour using only measurable pressure, volume, temperature and amount. At constant pressure, heating gives the particles more kinetic energy and the volume increases so the collision effect on the walls remains consistent.

Real gases approach the model under suitable conditions but deviate at very high pressure or low temperature. Particles are then closer: attractions can pull particles inward and lower the measured wall pressure, while the particles' own volume reduces the free space available for movement.

Do not treat ‘ideal’ as the description of every real gas or forget which assumptions fail. Keep this card on model assumptions and their limits; the numerical use of pV = nRT, unit conversion and Mr calculations belong to the neighbouring quantitative objective.