Simple Diffusion ExperimentsOxford AQA IGCSE Chemistry: Revision notes

Diffusion is the spreading out of particles from an area of higher concentration to an area of lower concentration, until they are evenly spread. Simple diffusion experiments provide direct evidence for the existence of particles, and show that particles are constantly moving.

The fact that a substance can spread through air or through a liquid without being stirred shows that its particles must be moving randomly and independently.

If a gas jar of orange-brown bromine (Br₂) vapour is connected to a gas jar of air, the bromine colour is seen to spread gradually into the air-filled jar, even though the two are stationary and not mixed by stirring.

• This shows bromine gas particles are moving and spreading into the air by diffusion
• The process is faster in gases than in liquids, because gas particles move faster and are further apart

Cotton wool soaked in ammonia solution (NH₃) and cotton wool soaked in concentrated hydrochloric acid (HCl) are placed at opposite ends of a long glass tube.

• Ammonia gas and hydrogen chloride gas both diffuse along the tube from their respective ends
• Where the two gases meet, they react to form a white ring of solid ammonium chloride smoke
• The ring forms closer to the hydrochloric acid end, because ammonia molecules are lighter and diffuse faster than the heavier hydrogen chloride molecules

A crystal of purple potassium manganate(VII) (KMnO₄) placed in a beaker of water will slowly dissolve and its purple colour will spread outward through the water over time, without any stirring.

• This shows that even in a liquid, particles move and spread from an area of high concentration (around the crystal) to low concentration (throughout the water)
• Diffusion in liquids is much slower than in gases, because liquid particles are closer together and move more slowly