States of Matter

• Diffusion is the net movement of particles from a region of high concentration to a region of low concentration, by random particle motion
• Happens in gases and liquids only. Solid particles vibrate in place and cannot migrate
• Requires no external energy input
• Faster at higher temperatures (particles carry more kinetic energy)
• Much faster in gases than in liquids (gas particles are further apart and move faster)

Diffusion in gases: the bromine demonstration

• A gas jar of red-brown bromine vapour is placed beneath an upturned jar of colourless air, separated by a glass cover
• Cover removed → bromine and air particles mix
• After a few minutes both jars show the same uniform pale-brown colour
• Mixing occurs even though bromine vapour is denser than air

Diffusion in liquids: the potassium manganate(VII) demonstration

• A small purple crystal of KMnO₄ is dropped into still water
• The colour spreads outwards through the water with no stirring
• After several hours the beaker is uniformly pale purple
• Much slower than gas diffusion (liquid particles are close-packed)