Solids, liquids, gases: at the microscopic level

An ice cube, liquid water and water vapour are all exactly the same substance: water molecules, H₂O. What differs between the three is not the nature of the molecules, but the way they are arranged and how they move in relation to one another.

What really happens

State	Arrangement of molecules	Movement
Solid	tightly packed, arranged in an orderly fashion	vibrate in place, do not move
Liquid	tightly packed but disordered	slide over one another
Gas	very far apart from one another	move freely throughout the available space

It is this microscopic organisation that explains the behaviour you observe every day: a solid retains its shape because its molecules are locked together. A liquid takes the shape of its container because its molecules can slide past one another, but remains incompressible because they remain bound together. A gas, on the other hand, fills the entire available volume: its molecules are so far apart that there is almost nothing but empty space between them, and it can easily be compressed.

A common misconception

Many pupils imagine that the molecules in a solid are stationary. This is incorrect: they vibrate constantly around a fixed position, as if each were attached to its neighbours by a spring. The more the solid is heated, the more these vibrations intensify, until they become strong enough to disrupt the order of the lattice: this marks the start of melting.

Remember the key idea of this chapter: a change of state is never a change of matter. The water molecules in vapour are exactly the same as those in an ice cube. Only their arrangement has changed.