Gas molecules move in constant random motion
Constant random motion
Gas molecules move all the time, in all directions, with no fixed pattern.
- In a gas the molecules are far apart compared with their size.
- They move quickly and randomly, travelling in straight lines between collisions.
- They change direction when they collide with each other or with the walls of their container.
- Because the motion is random, the molecules are not all moving the same way; at any instant they have a range of speeds and directions.
Temperature and average kinetic energy
Kinetic energy
Kinetic energy is the energy a particle has because it is moving.
- The temperature of a gas is related to the average kinetic energy of its molecules.
- If the temperature rises, the average kinetic energy increases and the molecules move faster on average.
- If the temperature falls, the average kinetic energy decreases and the molecules move more slowly on average.
- The word “average” matters: some molecules move faster and some more slowly, but a higher temperature means a greater average.
A higher temperature means a greater average kinetic energy of the gas molecules, so they move faster on average.
Gas pressure comes from collisions
Gas pressure
Gas pressure is the pressure a gas exerts because its molecules collide with the walls of the container.
- Gas molecules collide with the walls of their container, and each collision exerts a force on the wall.
- The combined effect of many collisions every second produces the pressure of the gas.
- A gas exerts pressure in all directions because its molecules move randomly in all directions.
Changing temperature at constant volume
- If a fixed amount of gas is heated in a rigid container so its volume stays constant, the temperature rises.
- The average kinetic energy of the molecules increases, so they move faster on average.
- They collide with the walls more often and hit them harder.
- So the pressure of the gas increases.
- If the same gas is cooled at constant volume, the molecules move more slowly, collide less often and less hard, so the pressure decreases.
Question: A sealed metal can contains air. The can is heated but its volume stays constant. Explain why the pressure of the air inside increases.
Model answer: The air molecules gain kinetic energy because the temperature increases, so they move faster on average. They collide with the walls of the can more often and with greater force, so the pressure inside the can increases.
- Do not say that heating a gas “creates pressure” or makes more molecules; heating does not make more gas.
- The pressure rises because the same molecules gain kinetic energy, move faster, and collide with the walls more often and harder.
- For an explanation, use a clear cause-and-effect chain: temperature increases →\rightarrow→ average kinetic energy increases →\rightarrow→ molecules move faster →\rightarrow→ collisions with the walls become more frequent and harder →\rightarrow→ pressure increases.
- Mention that the volume is constant when the question gives that condition.
- Describe the motion of the molecules in a gas.
- What is the temperature of a gas related to?
- What causes the pressure of a gas?
- Why does heating a gas at constant volume increase its pressure?
- What happens to the pressure if the gas is cooled at constant volume?
