7.3 Melting & Boiling

A state is not permanent. Heat a solid and it can become a liquid; heat the liquid and it becomes a gas. Heat is the switch — because heat changes how energetically the particles move.

Melting Point Heating a solid makes its particles vibrate more vigorously. At a certain point the vibrations grow so strong that particles break free of their fixed positions, the interparticle attractions weaken, and the solid turns into a liquid. The minimum temperature at which a solid melts into a liquid (at atmospheric pressure) is its melting point. Solids with weak attractions have low melting points; those with strong attractions have high ones.

Boiling Point Heat a liquid and its particles move faster still, drifting apart and weakening their attractions further, until they escape into the gaseous state. The temperature at which a liquid boils into vapour (at atmospheric pressure) is its boiling point. At the boiling point, vapour forms rapidly throughout the liquid — seen as bubbles.

Evaporation: Boiling’s Quiet Cousin Spilled water dries up even on a cool day — without boiling. That’s evaporation: vapour forms at all temperatures, not just the boiling point, but slowly and only at the surface. Boiling is fast and happens throughout the liquid; evaporation is gentle and happens only at the top.

Use the heater below: raise the temperature and watch a solid’s particles loosen into a liquid, then break free as a gas.

Science Talk Ice is a strange exception: its particles are actually farther apart than in liquid water (which is why ice floats!). For most substances, particles are a little farther apart in the liquid than in the solid. Talk through what melting does to interparticle distance and interparticle attraction — do they increase or decrease?