Gas molecules move in constant random motion
Definition
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
Definition
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.
Key Idea
A higher temperature means a greater average kinetic energy of the gas molecules, so they move faster on average.
Gas pressure comes from collisions
Definition
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.
Common Mistake
- 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.
Exam technique
- 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.
Self review
- 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?
