Movement of particles: Brownian motion and diffusion

In a nutshell

Brownian motion is observable evidence that particles move randomly in all directions. Diffusion is a process whereby particles move from an area of high concentration to low concentration. Temperature increases the movement of particles, so it makes Brownian motion more observable and also increases the rate at which diffusion happens.

Brownian motion

Particles in a fluid move randomly in all directions. Fluids are liquids and gases. This random motion is called Brownian motion and is named after the scientist who discovered it, Robert Brown.

Brown placed pollen grains in water and observed them under a microscope. He observed that the pollen grains appeared to be shaking around the water moving in random directions. At the time he couldn't explain his observations.

It was Einstein who later explained that the pollen grains were being moved by individual water particles as they collided with the much larger pollen grains.

Brownian motion is the term given to the random movement of larger particles being bombarded by smaller, faster moving particles.

Note: The Brownian motion experiment can also be done with smoke particles in air and milk fat particles in water.

Diffusion

Diffusion is the movement of particles from an area of high concentration to an area of low concentration. As it involves moving particles, diffusion only occurs in fluids.


Particles are concentrated in the corner.	Over time particles have diffused and spread out.

The particles are concentrated in the bottom left corner and over time they randomly move in all directions until they are all spread out.

Curiosity: Diffusion is how smells spread across a room.

Temperature

Increasing temperature increases the amount of energy particles have. This means that the higher the temperature the more the particles will vibrate and move around.

Therefore increasing the temperature will increase the rate of diffusion and will increase the observation of Brownian motion. Diffusion will happen much quicker and Brownian motion will observe particles moving around much quicker.