IB Physics: The Ideal Gas Laws — FAQ

Start by identifying which version of the equation you need — pV = nRT for moles or pV = NkT for molecule count — then convert every value to SI units: kelvin, pascals and cubic metres. Check whether pressure, volume or temperature is held constant, then substitute and solve, keeping your answer to sensible significant figures.

The ideal gas law is pV = nRT, where p is pressure in pascals, V is volume in cubic metres, n is the amount of substance in moles, R is the molar gas constant (8.31 J K⁻¹ mol⁻¹), and T is absolute temperature in kelvin. IB Physics also uses the equivalent form pV = NkT.

pV = nRT uses the number of moles (n) with the molar gas constant R = 8.31 J K⁻¹ mol⁻¹; pV = NkT uses the actual number of particles (N) with the Boltzmann constant k = 1.38 × 10⁻²³ J K⁻¹. They describe the same law, since R equals Avogadro's number multiplied by k.

Both — the ideal gas law sits within Theme B, The Particulate Nature of Matter, in the current DP Physics guide, and it's assessed at SL and HL alike. HL students go further, deriving gas pressure from kinetic theory and analysing molecular speed distributions in more mathematical depth.

Examiners expect four assumptions: molecules have negligible volume compared with the container, there are no intermolecular forces except during collisions, all collisions are perfectly elastic, and molecular motion is random with a range of speeds and directions. Miss one on a "state the assumptions" question and you typically lose a mark each.

Convert every temperature to kelvin before you touch the equation — this single step accounts for more lost marks in Paper 2 thermal questions than anything else. Then check pressure is in pascals, not kPa or atm, and volume is in cubic metres, not cm³ or litres, and confirm whether you've been given moles or molecule count.

The mistake I mark most often is forgetting to convert Celsius to kelvin, which wrecks every ratio-based calculation. Close behind is mixing up R and k, or pairing R with N instead of n. Third: treating a process as isothermal when the question actually states that pressure or volume — not temperature — stays constant.

On a pV diagram, each curve at fixed temperature is an isotherm where pV stays constant, so curves further from the origin represent higher temperatures. The area under the curve equals the work done by the gas; a vertical line means constant volume, so no work is done during that process.

Gas law questions turn up across all three papers. Paper 1's multiple-choice items usually test the equation and its variables directly; Paper 2 typically asks for a full calculation combined with a graph or real-gas scenario; Paper 3 sometimes links gas laws to experimental data analysis within the thermal physics context.

A standalone gas law calculation is usually worth 2-3 marks: correct substitution, correct rearrangement and a correct final answer with units. A full Paper 2 question combining the gas law with kinetic theory or a pV diagram can run to 6-8 marks spread across several sub-parts, so read every command term carefully.

Yes — the ideal gas law connects directly to kinetic theory, since temperature is proportional to the average kinetic energy of gas particles, and to thermodynamics questions about work done during expansion or compression. It also underpins the greenhouse effect sub-topic, where gas behaviour explains atmospheric energy transfer.

HL students meet the same ideal gas law as SL, but they're additionally expected to derive gas pressure from kinetic theory using momentum change at the container walls — a genuinely harder piece of mathematics. In my experience, SL students who know the equation cold handle calculations fine; it's the HL derivation that catches people out.

The physics itself is identical — both use pV = nRT and pV = NkT — but IB Physics places gas laws inside Theme B, The Particulate Nature of Matter, alongside the greenhouse effect and thermal energy transfer, giving your child more real-world context than the more isolated thermal physics module in most A-level specifications.

Look for resources offering worked past-paper questions on gas law calculations, a revision note that sets out both equation forms side by side, and topical worksheets mixing mole-based and molecule-based problems. On RevisionPrep, the DP Physics question bank and Revision Notes cover Theme B gas laws with mark-scheme-style worked solutions.