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Molecules in a gas

The motion of molecules in a gas can easily explain the way a gas behaves. You will probably have seen a model of a gas using small ball bearings to take the place of gas molecules.

The pressure of a gas is due to the many collisions of molecules with the walls of the container, the greater the number of collisions per second or the greater the velocity of the molecules the bigger will be the pressure of the gas.

So if we take a gas and heat it, the velocity of the molecules will get bigger and the pressure of the gas will also increase. If you heat a balloon very gently over a bunsen flame it is very easy to see this happening. If we squash a gas we are compressing the molecules into a smaller space and that means that they will hit the sides of the container more often and so the pressure will be greater - this result is predicted by Boyle’s Law. It is important to realise that the gas molecules will not all be moving with the same velocity, some will be moving fast and others slowly. However, most of them will be travelling near the average velocity and so there will be an average kinetic energy of a molecule. We believe that the temperature of a gas depends on the average kinetic energy of the gas molecules and so heating a gas will increase this energy.

Now think of two cylinders of gas, both at the same temperature and pressure but containing different gases. The average kinetic energy of the molecules of the two gases will be the same as their temperatures are equal but if one gas consists of heavier molecules than the other then the one with the lighter molecules will be moving more quickly.

Therefore molecules of hydrogen will be moving faster than molecules of oxygen at the same temperature because the molecules of hydrogen are much lighter. The velocity of air molecules at room temperature is about 300 m/s.

schoolphysics gas molecules animation

To see an animation of the change in motion of the molecules in a gas when it is heated click on the animation link.
© Keith Gibbs 2013