Explain how the motion of the molecules in a gas is related to both its…

The idea

Gas molecules move randomly at high speed, colliding with each other and with the container walls. Temperature measures the average kinetic energy of these molecules: heating a gas increases that average kinetic energy, so the molecules move faster. Pressure arises from the total force of molecular collisions on each unit area of the container wall. Faster molecules hit the walls more often and harder, so raising the temperature at fixed volume raises the pressure. For example, heating a sealed rigid can of gas makes its molecules faster, increasing the collision rate and force on the walls, so the pressure rises. Cooling reverses this: molecules slow, collisions become less frequent and less forceful, and pressure falls.

What earns the marks

• Gas molecules move randomly and rapidly, colliding with each other and with the walls of the container.
• Temperature is a measure of the average kinetic energy of the gas molecules, so a higher temperature means a higher average speed.
• Pressure is caused by the force of molecular collisions acting on the container walls per unit area.
• At constant volume, increasing temperature increases the average kinetic energy and speed of the molecules.
• Faster molecules collide with the walls more frequently and with greater change of momentum, so the force on the walls and the pressure increase.
• A complete explanation links temperature to average kinetic energy, then to collision rate and force, then to pressure.

Common mistakes and how to put them right

• Saying that molecules expand or get bigger when heated.

  Put it right by stating that the molecules themselves do not change size; their average speed and kinetic energy increase.

• Claiming that pressure is caused by molecules pushing each other rather than hitting the walls.

  Put it right by explaining that pressure comes from collisions with the container walls per unit area.

• Confusing temperature with total kinetic energy.

  Put it right by stating that temperature relates to the average kinetic energy of the molecules, not the total.

Exam tips

• Use the chain temperature → average kinetic energy → average speed → collision rate and force on walls → pressure.
• Refer explicitly to collisions with the walls per unit area, because that is what pressure measures.
• Use comparative language such as faster, more frequent and greater force rather than simply saying the molecules move more.