Liquids: a fixed volume, but no fixed shape

What does a liquid keep, and what does it give up? (Activity 7.4) Take three clean, dry containers of different shapes, label them A, B and C, and mark the 200 mL level on each.

Fill A with water up to the mark. Pour it carefully into B, and look. Then pour it from B into C, and look again.

The water takes the shape of each container in turn. Liquids have no fixed shape, because their particles are free to move about. But in every container the water still reaches the 200 mL mark. Liquids do have a definite volume. (If a container isn't clean and dry, a little water clings to its walls and the next reading comes out just under 200 mL.)

So a liquid's particles can move freely, but only within a limited space. Now slide your finger through some water in a shallow dish. You can, without breaking the water: you push it aside only for a moment, and it closes up again behind your finger. Try that with a solid! So in liquids the attractions are a little weaker than in solids, but still strong enough to keep the particles close together.

Heat a liquid enough and it boils. The temperature at which a liquid boils and turns into vapour at atmospheric pressure is its boiling point. The particles move so wildly that they break away from each other and escape into the gas state.

Liquids also turn into vapour below their boiling point. Spill water on a hot day of 35 °C and it dries up, though water boils at 100 °C. That slower process is evaporation.