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Molecules in gases


In a solid each molecule vibrates about a mean position. The hotter the solid the more violently the molecules vibrate.


In a gas the molecules are all moving about randomly and at high speed. At room temperature they travel at around 400 m/s. The molecules must be moving faster than the speed of sound because it is just these moving molecules that transmit sound. Heating the gas makes the molecules move around more violently while cooling the gas makes the molecules slow down.

If you keep cooling the gas the molecules move slower and slower and have less and less kinetic energy. In the end they would stop zero kinetic energy and so zero temperature. This temperature is called ABSOLUTE ZERO and it value is 273oC. We do not believe that it is possible to reach absolute zero although scientists have got close to this, down to 0.000 001 oC above that temperature.

As the molecules move around they collide with both each other and any solid object. If the gas is in a container the molecules collide with the walls of the container at high speed. It is these collisions that result in the pressure of the gas

If you compress the gas the molecules hit the walls of the container more often this means an increase in pressure.

If you heat the gas the molecules collide with the walls more violently this also increases the gas pressure.
The reverse is also true expansion and cooling result in a decrease in gas pressure.

Evidence for molecular motion

One of the best ways of showing that air molecules are moving is known as Brownian motion. Some smoke is blown into a small glass pot containing air and then viewed through a microscope. The air molecules collide with much larger smoke particles and if a strong light is shone on the box the smoke particles can be seen. They judder around as they are hit from all sides by the much smaller and invisible air molecules. The particles can be seen because light is scattered from them and they look like tiny stars.

(Compare with the polystyrene ball in the kinetic theory model apparatus using small ball bearings to represent air molecules).

If some bromine is released into a tube of air the colour of the bromine gas can be seen spreading slowly through the air this is diffusion. The air molecules affect the movement of the bromine molecules. If the experiment is repeated in an evacuated tube the spread of colour is virtually instantaneous.


© Keith Gibbs 2020