A latent heat graph (also called a heating curve) shows how temperature changes with time as a substance is heated. The key feature is that temperature remains constant during a phase change (melting or boiling) — this flat region represents the absorption of latent heat. Latent heat is the heat energy absorbed or released during a change of state without any change in temperature.
Latent heat graph shows flat lines at melting point and boiling point.
Flat line = latent heat absorption; temperature does not change during phase change.
Latent heat of fusion of ice = 334 kJ/kg (at 0°C).
Latent heat of vaporisation of water = 2260 kJ/kg (at 100°C).
Formula: Q = mL (latent heat); Q = mcΔT (sensible heat).
Vaporisation requires much more energy than fusion — molecules must fully escape intermolecular forces.
Heating Curve Description (e.g., Ice → Water → Steam):
Region 1 — Solid Heating: • Temperature rises as ice absorbs heat • Slope depends on specific heat capacity of solid
Region 2 — Melting (Flat Line at 0°C for ice): • Temperature stays at 0°C • Heat absorbed = Latent Heat of Fusion • All energy goes into breaking intermolecular bonds, not raising temperature • Latent Heat of Fusion of ice = 334 kJ/kg (3.34 × 10⁵ J/kg)
Region 3 — Liquid Heating: • Temperature rises from 0°C to 100°C • Slope: specific heat capacity of water (4200 J/kg°C)
Region 4 — Boiling (Flat Line at 100°C for water): • Temperature stays at 100°C • Heat absorbed = Latent Heat of Vaporisation • All energy goes into converting liquid to vapour • Latent Heat of Vaporisation of water = 2260 kJ/kg
Region 5 — Steam Heating: • Temperature rises above 100°C • Slope: specific heat capacity of steam
Graph Summary:
| Region | Description | Temperature Change |
|---|---|---|
| 1 | Solid heating | Rising |
| 2 | Melting (fusion) | Flat (0°C) |
| 3 | Liquid heating | Rising |
| 4 | Boiling (vaporisation) | Flat (100°C) |
| 5 | Gas/steam heating | Rising |
Key Formulae: • Q = mL (latent heat absorbed/released) – Q = heat (joules), m = mass (kg), L = specific latent heat (J/kg) • Q = mcΔT (sensible heat — when temperature changes) – c = specific heat capacity, ΔT = temperature change
Important Values: • Latent heat of fusion of ice: 334,000 J/kg (334 kJ/kg) • Latent heat of vaporisation of water: 2,260,000 J/kg (2260 kJ/kg)
Why is Latent Heat of Vaporisation much larger? • In vaporisation, molecules must overcome all intermolecular attractions and expand against atmospheric pressure — far more energy needed than just loosening bonds in melting.
A latent heat graph (heating curve) shows temperature vs time as a substance is heated. The graph has flat regions at the melting point (e.g., 0°C for ice) and boiling point (e.g., 100°C for water) — during these flat sections, temperature stays constant while latent heat is absorbed to change the state. Between flat sections, temperature rises as sensible heat is added.
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