Diffusion is just movement from high to low concentration, right? But why?

We've all learned that diffusion is the movement of a substance from an area of high concentration to lower concentration.

But why? What is the reason behind why particles choose to move down the concentration gradient in this way? 

It's all just a matter of statistics!

Random particle movement

Particles have energy and this energy make particles vibrate and move around. This movement is random and if one particle collides with another, they will change their course and continue to move in a different direction.

Concentration

Where lots of the same particle exist in the same space, we say it is highly concentrated. When there are fewer particles in the same space, the concentration is lower.

Chance of collision

In the highly concentrated area, the chance of particles colliding is high (think of the bumper cars at the fairground). As these particles collide, they change direction, some may go directly into the path of another particle and collide again, but, other particles may also be pushed into space (where there is a lower concentration of particles) and continue to travel collision-free. In this case, we say that there is a high concentration gradient (or a big difference in the concentration between 2 spaces).

As particles are continually knocked into the area of lower concentration (because there are fewer particles getting in their way), the difference in concentration from the highly concentrated area to the area of lower concentration means that the chance of collision becomes more similar. In this case, we say that the concentration gradient is more shallow.

This continues until there is no concentration gradient and the chance of collision is the same across the whole space. When this point is reached, we call it equilibrium.