Grade 8 physics · Thermodynamics
Melting and crystallization
A phase change rearranges particles. Energy enters or leaves the substance even when its temperature temporarily remains constant.
Learning goals
- Distinguish melting from crystallization using a particle model.
- Use $Q=\lambda m$ with SI units and a clear energy direction.
- Interpret the plateau of an ideal heating or cooling curve.
- Explain supercooling and nucleation without treating them as universal fixed thresholds.
1. Two directions of one phase transition
Melting is the transition from solid to liquid. The substance absorbs energy as its ordered solid structure breaks down. Crystallization (freezing for a liquid) is the reverse transition: a liquid forms an ordered solid and transfers energy to its surroundings.
At a specified pressure, a pure crystalline substance has a definite equilibrium melting temperature. Solid and liquid may coexist there. The melting point depends on both the substance and pressure; impurities can broaden or shift the transition. Glass and many polymers are amorphous and usually soften through a temperature range instead of showing one sharp melting plateau.
During either direction, particles do not vanish or change chemical identity. Their arrangement and interaction energy change. Heating does not mean that every supplied joule must increase temperature.
2. Specific latent heat of fusion
For a mass $m$ that changes completely between solid and liquid at its phase-change temperature, the magnitude of transferred energy is