Gas pressure is caused by particles colliding with the walls of their container. Each collision pushes on the wall, and the pressure is the total force of these collisions on each unit of area. Questions ask you to explain a pressure change when a gas is heated, compressed or expanded.
This lesson sits in the module on particle models and changes of state. It follows on from reading a heating curve, where the particles were in liquid or solid states.
How do collisions create pressure?
In a gas the particles are far apart and move randomly and quickly in straight lines until they collide. When a particle hits a wall, it bounces back and pushes on the wall.
One push is tiny. A huge number of pushes each second make a steady force.
Two things decide the pressure:
- how often the particles hit the wall, and
- how hard each particle hits the wall.
A good explanation names both when they change. If only one changes, name that one.
What happens when temperature or volume changes?
Heating a gas in a sealed, rigid container (constant volume). The particles gain kinetic energy and move faster. They hit the walls more frequently and with more force, so the pressure increases.
Compressing a gas at constant temperature. The particles have the same average speed, so each collision is equally hard. The volume is smaller, so the particles reach the walls more often. There are more collisions each second on each unit of area, so the pressure increases.
Expanding a gas at constant temperature. The opposite: fewer collisions each second on each unit of wall, so the pressure falls.
Your syllabus and teacher may also use the relationship that, for a fixed mass of gas at constant temperature, pressure × volume stays constant. Check the current syllabus on the Cambridge page for what is expected in your course. The particle explanation matters either way.
Worked example
The numbers below are invented for practice.
A sealed syringe holds 600 cm³ of air at a pressure of 100 kPa. The plunger is pushed in slowly until the volume is 300 cm³. The temperature stays the same.
Question: Explain the pressure change, and find the new pressure if your course uses pressure × volume = constant.
Step 1, explain with particles. The volume is halved. The particles move at the same speed, but they now have less space. They hit the walls twice as often on each unit of area, so the pressure increases.
Step 2, set up the relationship. Pressure × volume is constant, so 100 × 600 = p × 300.
Step 3, calculate. 100 × 600 = 60 000. p = 60 000 ÷ 300 = 200 kPa.
Step 4, check. Halving the volume should double the pressure. 100 × 2 = 200, which matches.
If the question asks only for an explanation, the first step is enough. Adding a number does not replace the collision reasoning.
A second situation uses temperature in kelvin. If a sealed rigid container of gas is heated from 27 °C to 327 °C, then 27 + 273 = 300 K and 327 + 273 = 600 K. The kelvin temperature has doubled, so the pressure doubles, as long as the volume and amount of gas stay the same.
Use this only if your course includes the kelvin scale.
The mistake to watch for
A frequent error is to say the particles expand or get bigger when the gas is heated.
Mistaken answer: “The pressure rises because the particles expand when they are heated, so they push the walls out.”
The particles themselves do not change size. They move faster.
A second common slip is to say that pressure comes from particles hitting each other. Collisions with the wall cause pressure.
The correction: “The particles gain kinetic energy and move faster, so they hit the walls more often and with greater force, increasing the pressure.”
Check yourself
1. Explain why a sealed can of air, left in a hot car, is more likely to burst than one left in a cool room.
Show answer
In the heat the particles have more kinetic energy and move faster. They hit the inside of the can more often and with greater force, so the pressure is higher and the can is more likely to fail.
2. A gas is at 120 kPa in 500 cm³. The volume is reduced to 250 cm³ at constant temperature. What is the new pressure?
Show answer
120 × 500 = 60 000. 60 000 ÷ 250 = 240 kPa. Check: halving the volume doubles the pressure, and 120 × 2 = 240.
3. True or false: “When a gas cools in a rigid container, the particles hit the walls with the same force as before.” Explain.
Show answer
False. Cooling reduces the kinetic energy, so the particles move slower. They hit the walls less often and with less force, so the pressure falls.
Where this leads next
The last lesson in this module asks you to separate a change of substance from a change in spacing. Use the mixed particle practice when you want a spread of these questions together.
Explaining with a cause, a change and a result is a habit that our teachers build in online one-to-one Chemistry tuition.