Diffusion and gas pressure

Smelling the perfume

Someone opens a bottle of perfume at the front of the classroom. A minute later, you can smell it at the back. Nobody stirred the air. The particles did all the work by themselves.

What is diffusion?

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. It happens because the particles are always moving randomly. It needs no stirring and no energy supply. It carries on until the particles are spread out evenly.

Why does it go one way?

Particles move randomly in every direction, so some move from the crowded region to the empty region. But there are more particles in the crowded region, so more of them move out than move in. That is why we say the net movement is from high concentration to low concentration.

Lighter gases diffuse faster

At the same temperature, all gas particles have the same average kinetic energy. So the lighter particles must be moving faster than the heavier ones. So the lower the relative molecular mass, the faster the gas diffuses. And a higher temperature makes every gas diffuse faster, because the particles gain kinetic energy.

Mr and speed

Here are four gases. Hydrogen has Mr two, the lowest, so it diffuses fastest. Methane is sixteen and ammonia seventeen. Carbon dioxide has Mr forty-four, the highest, so it diffuses most slowly.

The classic experiment

Cotton wool soaked in ammonia is placed at one end of a long tube, and cotton wool soaked in hydrochloric acid at the other. The two gases diffuse towards each other, and meet where a white ring of ammonium chloride forms. The ring is nearer the hydrogen chloride end. Ammonia, Mr seventeen, is lighter than hydrogen chloride, Mr thirty-six point five, so it has travelled further.