5.15-5.17 - Gas pressure, absolute zero and Kelvin scale

Random gas motion

Molecules in a gas are far apart compared with the size of each molecule. They are not fixed in place; they move rapidly and randomly throughout the space available to them.

Random motion means the molecules do not all travel in one neat direction. At any instant, different molecules are moving in different directions and at different speeds, so a gas fills the container rather than forming a tidy layer.

The word molecule is used in the specification, but the same particle-model explanation also works for gases made of individual atoms. What matters here is that the gas particles are moving and repeatedly colliding with each other and with the walls of the container.

Because the motion is random, a closed container is hit on all sides. The gas does not only push down like a solid object resting on a surface; it can push on the top, bottom and sides of the container.

Wall collisions and pressure

When a gas molecule collides with a wall, the wall changes the molecule's motion. That means the molecule and the wall exert forces on each other during the collision.

One molecule gives a tiny force for a tiny time. A real sample of gas contains an enormous number of molecules, so the wall receives very many impacts every second. The total effect is a steady force on the wall.

Pressure is linked to this force:

For a gas in a container, the force comes from molecular impacts on the wall. The same total force spread over a larger wall area gives a smaller pressure, while a larger total force on the same area gives a larger pressure.

So the particle explanation for gas pressure is:

This lesson is only asking for the mechanism. Later gas-law work uses this idea to compare how pressure changes when temperature or volume changes.

Absolute zero

Temperature is connected with the random thermal motion of particles. Cooling a gas takes energy away from that motion, so the molecules move more slowly, hit the walls less often and less hard, and the gas pressure falls. This cannot go on forever: once the random thermal motion has been reduced as far as it can go, there is nothing left to take away, so there must be a lowest possible temperature.

Absolute zero is that lower limit of temperature. In Edexcel International GCSE Physics, you should use:

$absolute zero = -273 {}^\circ\mathrm{C}$

Experiments point to the same value. If a fixed volume of gas is cooled and its pressure is plotted against its temperature in ${}^\circ\mathrm{C}$, the points lie on a straight line. Extending that line downwards, it reaches zero pressure at about $-273 {}^\circ\mathrm{C}$, for any gas. (A real gas turns into a liquid before it gets that cold, so the line is extended beyond the measurements.)