Physics · 2.2.3 Melting, boiling & latent heat · Paper 6 practical
Changes of State. Melt it.
Heat a substance from solid to gas and watch the temperature–time graph. Temperature stays constant during melting and boiling (energy goes into latent heat). Add an impurity to see how it lowers the melting point and raises the boiling point.
0625 Topic 2.2.3 — Melting & boiling
Specific latent heat
Effect of impurities
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θ −20 °C · State solid
Variables
—
0%
60
Live readouts
Temperature θ
−20 °C
State of matter
solid
Melting point
0 °C
Boiling point
100 °C
During a change of state the temperature is constant — energy supplied becomes latent heat, breaking bonds rather than raising temperature.
θ vs t — heating / cooling curve
📋 Method (Cambridge ATP procedure)
- Place crushed solid (e.g. naphthalene) in a boiling tube with a thermometer; stand the tube in a water bath for even heating.
- Heat gently and record the temperature every 30 s while stirring.
- Continue until the solid has fully melted and the temperature begins to rise again.
- For cooling: heat the substance to a liquid, then let it cool, recording temperature every 30 s.
- Plot temperature against time. The flat region (plateau) marks the melting/freezing point.
Latent heat: during the plateau, energy is supplied/removed but temperature is constant — this energy is the latent heat of fusion (or vaporisation).
⚠ Effect of impurities & precautions
- Impurities lower the melting point and broaden the melting range (the plateau is no longer perfectly flat).
- Impurities raise the boiling point (e.g. salt water boils above 100 °C).
- Even heating — use a water bath, not a direct flame, to avoid overheating one side.
- Stir continuously for a uniform temperature.
- Thermometer position — bulb in the substance, not touching the glass.
🎯 Syllabus reference (0625)
- 2.2.3 Melting, boiling & condensation — describe melting and boiling in terms of energy input without a change in temperature; describe condensation and solidification in terms of energy release; distinguish boiling from evaporation.
- Specific latent heat — define and use E = mL.