📦 A sealed box
Air molecules inside a sealed box hit the walls all the time. Each collision pushes the wall outwards a tiny amount. The walls feel one steady push, not separate hits, because there are so many collisions.
Ideal Gas Molecules » Gas Molecules and Pressure
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 5.15, 5.18
- How gas molecules move around at random
- How molecules hitting a wall create a force and a pressure
- Why a gas pushes on every part of its container
- Why a higher temperature means faster molecules
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Unlock This CourseGas molecules never stop moving
The air around you is a mixture of gases. Each gas is made of tiny particles called molecules. In the lesson Particles in Solids, Liquids and Gases you saw that the particles in a gas are far apart and move quickly. This lesson uses that idea to explain something you can feel: the pressure of a gas.
The molecules in a gas are moving all the time, and they move randomly. A molecule travels in a straight line until it collides with another molecule or with the wall of the container. Then it bounces off and heads in a new direction. There is no pattern to the directions, and the molecules do not all move at the same speed. Some are fast, some are slow, and their speeds keep changing as they collide.
Key terms:
- Random motion: movement in any direction, with no pattern, that keeps changing as the molecules collide.
- Molecule: a tiny particle of a gas. A molecule of oxygen has two oxygen atoms joined together.
- Average speed: the typical speed of all the molecules in a gas taken together. Individual molecules can be faster or slower.
How a gas makes a force on the wall
Because the molecules move at random, some of them are always heading towards a wall of the container. When a molecule hits the wall it bounces back. During the collision the molecule pushes on the wall, and the wall pushes back on the molecule. So each collision exerts a tiny force on the wall.
One molecule gives a force far too small to notice. But a container of gas holds an enormous number of molecules, and each one hits the walls again and again. Huge numbers of collisions happen every second, spread over the whole area of the wall. These tiny forces add up to a steady push on the wall. This push on each part of the wall is the pressure of the gas.
Remember from the lesson Pressure that pressure is the force acting on each unit of area. Here the force comes from the collisions, and the area is the area of the wall they hit.
⚽ A football
A football feels hard because of the gas inside it. Huge numbers of molecules hit the inside of the ball every second and push it outwards. The ball keeps its shape because of these collisions.
Drawing it
You already know from the lesson Pressure in Liquids and Gases that a gas pushes on every wall in every direction. To show the collisions in an exam, draw a box. Draw a few dots inside it for the molecules, each with a short arrow showing the direction it is moving. Where a dot reaches a wall, draw an arrow pointing outwards on the wall to show the force.
Why temperature changes the speed
When a gas is heated, energy is transferred to it. The molecules gain kinetic energy, so they move faster. An increase in temperature results in an increase in the average speed of the molecules. Some molecules will still be slower than others, but on average they are quicker.
It also works the other way. If a gas cools down, the molecules lose kinetic energy, so their average speed goes down.
| Cool gas | Hot gas | |
|---|---|---|
| Kinetic energy of the molecules | Lower | Higher |
| Average speed of the molecules | Lower | Higher |
| Direction of movement | Random | Still random |
The molecules are not more numerous, and they do not get bigger. Only their speed changes. The motion is still random.
Worked example
Explain, using ideas about molecules, how the air in a sealed box exerts a force on the walls of the box.
Step 1: The air molecules move quickly and at random.
Step 2: Molecules collide with the walls of the box.
Step 3: Each collision exerts a tiny force on the wall.
Step 4: There are huge numbers of collisions every second, so the forces add up to a steady force on the wall. Over the area of the wall, this force gives the pressure.
Common mistakes
- Saying that the molecules in a gas push on each other to make the pressure. The pressure on a wall comes from molecules hitting the wall.
- Saying one collision makes the pressure. It takes huge numbers of collisions every second.
- Saying that gas molecules expand or get bigger when they are heated. The molecules stay the same size. They move faster.
- Saying that every molecule has the same speed. The speeds vary, so we talk about the average speed.
Exam-style question
A sealed flask contains air.
(a) Describe the motion of the molecules of air in the flask. [2 marks]
(b) Explain how the air in the flask exerts a pressure on the walls of the flask. [3 marks]
(c) The flask is warmed. Explain what happens to the average speed of the molecules. [2 marks]
Model answer
(a) The molecules move randomly (1). They move quickly in straight lines until they collide with other molecules or the walls (1).
(b) Molecules collide with the walls of the flask (1). Each collision exerts a tiny force on the wall (1). There are huge numbers of collisions every second, and together they exert a force over the area of the wall, which gives the pressure (1).
(c) Warming transfers energy to the molecules, so their kinetic energy increases (1). The average speed of the molecules increases (1).
Exam tip
In a pressure question, use all three ideas: molecules hit the wall, each collision gives a tiny force, and there are huge numbers of collisions. You need all three for full marks.