Solids: packed tight, and how they melt

Activity 7.3. Collect a piece of iron or an iron nail, a piece of rock salt, a stone, a piece of wood, a key and a piece of aluminium. Look at their shapes and sizes, and try hammering them. In which do you think the particles are held most strongly?

All six are solids, and every one keeps a definite shape and a definite volume. That's because in a solid the particles are packed tightly and the attractions between them are very strong.

Those strong forces keep each particle in a fixed place. It can vibrate to and fro about its own spot, like a child frozen in a game of statue who can still wobble, but it can't move past its neighbours. That's why a solid can't flow, and is very hard to squash.

Now heat a solid. Its particles vibrate harder and harder, until they shake so much that they start to leave their places. The attractions weaken, and the solid turns into a liquid. The temperature at which this happens is the solid's melting point: the lowest temperature at which it melts into a liquid at atmospheric pressure.

In a liquid the particles are usually a little farther apart than in the solid. Ice is an exception: its particles are farther apart than in water, which is why ice floats.

Different solids hold their particles with different strengths. Weak attractions give a low melting point; strong ones a high melting point.