How much space is between the particles

How much space is there between particles? Activity 7.6 uses a syringe without a needle. Pull the plunger right out, put your thumb firmly over the open end so no air escapes, and push the plunger slowly in.

It goes a long way in: the air is squeezed into a smaller volume. So in a gas the particles have a lot of space between them, and pressure can push them closer. Let go and the particles spread out again, pushing the plunger back.

Now try it with water in the syringe. It hardly moves at all. Water is practically incompressible: its particles are already close together.

Activity 7.7 looks for space in a liquid another way. Half-fill a glass with water and mark the level A. Add two teaspoons of sugar and mark the new level B. Stir until the sugar dissolves, guess what will happen, then mark the final level C.

Adding sugar pushes the level up to B. But once it dissolves, the level comes back down a little, to C. The solution takes up less room than the water and sugar did separately. The sugar particles have slipped into the spaces already there between the water particles. The same happens with salt or glucose. But sand doesn't dissolve: its grains settle at the bottom and take up room of their own, so the level just goes up.

And in a solid? Its particles are packed very close and held by strong forces, yet there are still tiny spaces between them (Fig. 7.12a). You might think those spaces are full of air. They are not: they hold nothing at all.

One word of warning: particle is used in other ways too. The Suspended Particulate Matter in air pollution means tiny bits of dust, and each of those is itself made of enormous numbers of constituent particles.