IB Chemistry SLTopic 1 — The Behaviour of Ideal GasesPaper 1 & 2Core idea~8 min read
Real Gas Behaviour
The ideal gas model makes two convenient assumptions that aren’t quite true. Real gases have particles that take up space and attract one another — so under some conditions they drift away from PV = nRT. Knowing when and why is a classic exam point.
📘 What you need to know
The ideal gas equation assumes particles have negligible volume and no intermolecular forces.
Real gases deviate most at low temperature and high pressure.
At high pressure the particles’ own volume becomes significant.
At low temperature the attractions between particles become significant.
Both effects mean real gases don’t quite obey PV = nRT under those conditions.
When real gases deviate
The two assumptions behind PV = nRT — that particles take up no space and feel no attractions — are good approximations most of the time. But they break down under two conditions:
Low temperatures: particles move slowly, so the weak attractions between them start to matter.
High pressures: particles are squeezed close together, so their own volume is no longer negligible.
Real gases deviate most at low temperature and high pressure — the 200 K curve strays furthest from the ideal line; 1000 K stays closest.
The volume assumption
An ideal gas is assumed to have particles that take up no space at all. That’s fine when they’re far apart — but at high pressure the particles are pushed close together, and the space they physically occupy becomes a real fraction of the container. Less free space for movement means the gas no longer follows the ideal law.
At high pressure the particles are crowded together, so the space they take up is a significant fraction of the container.
The attraction assumption
An ideal gas is also assumed to have no forces between particles. In reality there are weak attractions. At low temperatures the particles move slowly enough for those attractions to take hold — pulling particles slightly towards each other reduces how hard and how often they hit the walls, so the real pressure is lower than the ideal equation predicts.
Notice the neat symmetry: high pressure exposes the volume assumption, and low temperature exposes the attraction assumption. Both are minimised — and the gas behaves most ideally — at low pressure and high temperature.
💡 Exam tip
If asked when a real gas behaves most like an ideal gas: low pressure and high temperature.
Link each condition to the right assumption — high pressure → particle volume; low temperature → intermolecular attractions.
The ideal gas equation and gas constant are both in the IB data booklet.
That completes The Behaviour of Ideal Gases — and with it, the whole of Topic 1, Models of the Particulate Nature of Matter. Next you’ll move into Topic 2, Models of Bonding & Structure.
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