Diffusion in different media
Diffusion is the spontaneous passing of molecules of one substance into another substance in contact with it, and of the second into the first. The spreading of spilled perfume in a room and sugar dissolving in tea are examples. Although perfume vapour molecules move at several hundred metres per second, they collide with air molecules very many times and change direction, so the smell reaches the far side of the room not at once but after some time. In liquids diffusion is slower: a coloured dye dropped into water spreads gradually through it. In solids diffusion is very slow: well-polished lead and gold plates pressed together and kept at room temperature for several years penetrated each other by about 1 mm. When the temperature rises, molecules move faster and diffusion speeds up: sugar dissolves faster in hot tea than in cold. Diffusion is important in nature: oxygen passes into the blood in the lungs by diffusion, and poisonous gases spread in the air.
“Watching diffusion” (with adult supervision): pour cold and warm (not boiling, 40–50 °C) water into two clear glasses; add one drop of food colouring to each without stirring and sketch every 2 minutes how far the colour has spread. Safety: do not use boiling water; do not give potassium permanganate crystals to children — use food colouring instead; do not taste the water.