Gas laws
Pressure emerges from particle collisions. Squeeze the piston or heat the gas and watch it climb.
Pressure vs volume
Readouts
What's happening
Gas pressure is nothing but the drumming of particles on the walls. Heating makes them faster (more and harder hits), compressing packs the same hits into less wall. The measured pressure (counted from actual wall collisions and smoothed) tracks the ideal-gas prediction P = nRT/V, shown as the curve on the graph.
- Ideal gas equationPV = nRT
On the syllabus
- GCSE Physics AQA 4.3.3particle model and gas pressure
- GCSE and A-Level Chemistry and Physics AQA 3.6.2the ideal gas equation and kinetic theory
Challenge
Predict first: halve the volume at constant temperature: does the pressure double exactly? Watch the measured point against the ideal curve.
Yes. At constant temperature P ∝ 1/V (Boyle's law), so halving the volume exactly doubles the ideal pressure; the measured point jitters around it.
Learn this properly
Lessons from GCSE Combined and Separate Science on this topic: worked steps, then exam-style questions with new numbers every time.
- Density and the particle modelParticle model, density and gas pressureFree
- Gases: temperature and pressureParticle model, density and gas pressurePlus
- Pressure and volume of a gasParticle model, density and gas pressurePlus
See the whole courseThe full course, topic tests and mock papers come with Plus. See Plus
FAQ
What is Boyle's law?
At constant temperature the pressure of a fixed amount of gas is inversely proportional to its volume, so PV stays constant. Halve the volume and the pressure doubles.
Why does heating a gas raise its pressure?
Hotter particles move faster, so they hit the walls more often and harder. At constant volume the pressure is proportional to the absolute temperature in kelvin.
Why must temperature be in kelvin?
The gas laws are proportional to absolute temperature, which starts at absolute zero (0 K = −273.15 °C). Using °C would give zero or negative pressures at ordinary temperatures.
How do I find the new temperature of a gas with the combined gas law?
Rearrange P₁V₁/T₁ = P₂V₂/T₂ to T₂ = P₂V₂T₁/(P₁V₁), with temperatures in kelvin. Gas at 100 kPa, 2.0 L and 300 K squeezed to 1.5 L at 150 kPa ends at T₂ = 150 × 1.5 × 300 / (100 × 2.0) = 337.5 K, which is 64.4 °C. Pick Temperature T₂ under Solve for to see each step.