Pressure & Temperature in Gases
Section: 1. States of Matter | Syllabus: Cambridge IGCSE Chemistry (0620)
What Causes Gas Pressure?
Gas particles are in constant, random motion and are constantly colliding with the walls of their container. Each collision exerts a tiny force on the wall - the combined effect of billions of these collisions every second is what we measure as gas pressure.
- Gas pressure is not caused by the weight of the gas - it is caused by particles hitting the container walls
- More frequent collisions, or harder collisions, increase the pressure
Gas pressure is the combined effect of billions of particle collisions with the container walls every second
Effect of Temperature on a Gas
Heating a gas gives its particles more kinetic energy, so they move faster. What happens next depends on whether the gas is free to expand:
- If the volume is fixed (a sealed, rigid container): faster particles hit the walls more often and with greater force, so the pressure increases
- If the pressure is kept constant (e.g. a gas in a syringe with a freely-moving piston): the container must expand so that the faster particles still collide with the walls no more often than before - the volume increases, and the particles become further apart on average
Cooling a gas has the opposite effect: particles slow down, so pressure falls (fixed volume) or volume falls (fixed pressure).
At constant pressure, heating a gas pushes the piston out - the volume increases so particles can keep colliding with the walls at the same rate
Worked Example: Heating a Gas at Constant Pressure
- Question: a sample of gas is held in a syringe with a freely-moving piston, so the pressure of the gas cannot change. The gas is heated. Describe and explain what happens to the volume of the gas and to the separation of its particles. [2]
- Answer: the volume increases - heating makes the particles move faster, so they push the piston outward to keep colliding with it at the same rate/force (keeping pressure constant) [1]; because the same number of particles now occupy a larger volume, the particles become further apart on average [1]
Effect of Volume (Pressure) on a Gas
At a constant temperature, particles keep the same average speed. Squeezing a gas into a smaller volume forces the same number of particles into a smaller space:
- Particles have less distance to travel between collisions with the walls
- Collisions with the walls become more frequent
- The pressure increases
Increasing the volume available to a gas has the opposite effect - collisions become less frequent, so the pressure decreases.
Explaining Everyday Observations
- Aerosol cans warn against heating: the can has a fixed volume, so heating makes the gas particles inside collide with the walls more often and more forcefully, increasing the pressure until the can could burst
- A balloon shrinks in a freezer: the cold air makes the gas particles inside move slower, so they exert less pressure; the flexible balloon skin is pushed in by the outside air pressure until it contracts
- A sealed crisp packet swells at high altitude: the lower outside air pressure at altitude means the gas trapped inside the packet is no longer balanced by as much outside pressure, so the packet expands
Common Mistakes
- ❌ Thinking gas particles get bigger or heavier when a gas is heated
- ✓ the particles themselves do not change size or mass - they simply move faster, so they collide with the container more often and with more force
Interactive revision notes, videos and practice questions load below.