7.5 Particles Always Moving
Particles are never still — they are in constant motion, jostling and spreading. Two everyday observations make this restless movement visible.
Activity 7.8 — A colour that spreads itself Drop a single grain of potassium permanganate into still water. At first, pink streaks creep out from the grain; given time, the whole tumbler turns evenly pink — with no stirring at all. The ever-moving water particles pull particles off the grain and knock them all through the liquid until the colour is uniform.
Diffusion The spontaneous spreading and mixing of the particles of one substance among those of another is called diffusion. It happens because particles are in constant motion. Diffusion is why a drop of colour fills a glass of water, and why a smell travels across a room — all without anyone stirring or fanning.
Heat speeds it up
Think Like a Scientist — Hot, warm, cold Drop a grain of potassium permanganate into hot, room-temperature, and ice-cold water at the same time. The colour spreads fastest in the hot water, slower at room temperature, and slowest in the cold. Heating makes the particles move faster, so diffusion speeds up.
Move the temperature slider and watch how quickly the colour spreads.
Even gases spread
Activity 7.9 — A scent across the room Light an incense stick in one corner of a room. At first you smell it only nearby; minutes later the fragrance has reached every corner. The scent particles, knocked along by the constantly-moving air particles, diffuse through the whole room. (It’s exactly why you can smell perfume from across a room.)
The Big Picture: Heat Decides the State Pulling it together: matter is made of particles held by attractions, and it is the particles’ thermal (heat) energy that decides the state. Low heat energy → particles stay close, strong attraction, only vibrating → solid. More heat overcomes some attraction → particles move within a space → liquid. Enough heat to overcome attraction entirely → particles fly free → gas.