1.3 Diffusion

Prior knowledge: You will need to have an understanding of how the states of matter differ, in terms of their particles.

Diffusion in liquids

When a drop of ink is placed into a beaker of water, the ink diffuses slowly throughout the whole volume of the water. It does this because both the water molecules and the ink particles are constantly moving, colliding with each other and randomly changing direction. It follows that the hotter the water is, the faster the particles will be moving around and the faster the ink will diffuse throughout the volume.


Diffusion in gases

To demonstrate diffusion in gases, a long glass tube is set up with cotton wool soaked with hydrochloric acid at one end, and cotton wool soaked with ammonia at the other end.

NH4Cl smoke

The hydrogen chloride and the ammonia gases diffuse along the tube from either end, because the particles are constantly, randomly moving. Where they meet, they react forming a white ‘smoke ring’ of ammonium chloride.

ammonia + hydrogen chloride  ammonium chloride

The smoke ring is not formed in the middle of the tube, but nearer to the end with the hydrochloric acid.  This tells us that the ammonia molecules travel further than the hydrogen chloride molecules in the same amount of time, in other words they diffuse more quickly.  This is because they are lighter: ammonia has a mass of 17 units while hydrogen chloride has a mass of 36.5 units.