Changes of state

A substance can change between the solid, liquid and gas states. A change of state is a physical change: the substance remains the same substance, but the arrangement and motion of its particles change.

In a solid, particles are held in fixed positions and vibrate. In a liquid, particles are close together but can move past one another. In a gas, particles move freely and are much farther apart.

Melting and freezing

Melting is the change from solid to liquid. As the substance gains energy, its particles move more and can overcome enough of the forces holding them in fixed positions. They can then move past one another as a liquid.

Freezing is the change from liquid to solid. The particles lose energy and move less. They become held in fixed positions, although they continue to vibrate.

Boiling and evaporation

Both boiling and evaporation change a liquid into a gas. In each case, particles gain enough energy to separate from the liquid and move freely as gas particles.

• Boiling happens throughout the liquid, forming bubbles of gas within it.
• Evaporation happens at the surface, where particles escape from the liquid into the gas state.

Condensation

Condensation is the change from gas to liquid. Gas particles lose energy and slow down. They come closer together and form a liquid, in which the particles can still move past one another.