IB Chemistry SLTopic 1 — The Behaviour of Ideal GasesPaper 1 & 2Core idea~7 min read
Ideal Gases
An “ideal gas” is a simplified model — tiny particles zipping about with no forces between them. Real gases aren’t quite like that, but the model works beautifully at low pressure and high temperature, and it’s the foundation of every gas calculation you’ll do.
📘 What you need to know
The kinetic theory pictures gas particles in constant, rapid, random motion.
An ideal gas follows all its assumptions: particle volume is negligible, there are no forces between particles, and all collisions are perfectly elastic (no kinetic energy lost).
Temperature is directly proportional to the average kinetic energy of the particles.
Real gases only approach ideal behaviour at low pressure and high temperature.
The volume a gas occupies depends on its pressure (P) and temperature (T).
The kinetic theory of gases
The kinetic theory explains how gases behave by making a few key assumptions about the particles:
Gas molecules are in constant, rapid, random motion.
The volume of the molecules themselves is negligible compared with the volume of the container.
There are no forces between the particles — they don’t attract or repel each other.
All collisions are perfectly elastic: no kinetic energy is lost when particles collide.
The temperature of the gas is directly proportional to the average kinetic energy of its particles.
Ideal vs real gases
A gas that obeys all of these assumptions is called an ideal gas. No real gas does so perfectly — but under the right conditions they come very close.
Real gases behave most like ideal gases at low pressure (particles far apart, so their own volume barely matters) and high temperature (particles moving fast, so attractions barely matter).
The two “ideal” assumptions that real gases break are exactly the two you’ll meet again in the Real Gases note: particles do take up some space, and they do attract each other a little. Low pressure and high temperature are the conditions that make both effects negligible.
Elastic vs inelastic collisions
The idea of a perfectly elastic collision is central. In an elastic collision the total kinetic energy is conserved — particles bounce off each other and move apart with no energy lost. In an inelastic collision, energy is lost and the particles may even stick together.
In an elastic collision particles bounce apart with no loss of kinetic energy; in an inelastic one they lose energy and tend to stick together. Ideal gas collisions are perfectly elastic.
💡 Exam tip
Learn the assumptions as a list — questions often ask you to state the conditions under which a real gas behaves ideally (answer: low pressure, high temperature).
“Perfectly elastic collisions” and “negligible particle volume” are the two phrases examiners look for.
Up next: Molar Gas Volume — how pressure, volume and temperature are linked, through Boyle’s and Charles’ laws.
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