Restless Particles: Why Gases Push

Why it matters

Open a perfume bottle in one corner of a room and within seconds someone at the far wall smells it. No fan, no breeze — just trillions of restless molecules sprinting, colliding and spreading into every bit of free space.

Predict first

A sealed metal can of air is heated, so its volume cannot change. What happens inside?

Heating raises the average kinetic energy of the molecules. They move faster, strike the walls more often and with greater momentum — so the pressure of the trapped gas rises even though its volume stays fixed.

• The molecules move faster and hit the walls harder — pressure rises — correct
• The molecules themselves swell and grow bigger
• Nothing changes, because the can is sealed

What you do

Slide the temperature through all three states. Past 100°C, watch the particles break loose, race in every direction and hammer the walls — that non-stop hammering is gas pressure.

Check yourself

Why does a gas exert pressure on the walls of its container?

Gas molecules move randomly at high speed and collide with each other and with the walls. The force these collisions exert per unit area is the gas pressure — and it acts equally in all directions.

Why can a gas be squashed (compressed) so easily, while a solid cannot?

Inter-molecular spaces in a gas are huge compared to the molecules themselves. Applying pressure simply pushes the molecules closer together into that empty space, so the volume shrinks — that is why gases are highly compressible.

According to the kinetic theory, the average kinetic energy of gas molecules is directly proportional to…

The average kinetic energy of a particle is directly proportional to its absolute temperature. Heat a gas and its molecules speed up; cool it and they slow — in theory ceasing entirely at absolute zero, −273°C.

• Its fast-moving molecules constantly collide with the walls — correct
• The weight of the gas presses down on the bottom only
• Gas molecules stick to the walls and pull on them
• There are large empty spaces between gas molecules — correct
• Gas molecules are softer than solid particles
• Some gas molecules are destroyed when you press
• the absolute (kelvin) temperature of the gas — correct
• the volume of the container
• the number of molecules present