Specific Latent Heat
Year 10 (IGCSE) 🌡️ Energy & Thermodynamics Distinguish latent heat of fusion and vaporisation; use Q = mL.
🧊 Latent Heat and Phase Changes
When a substance changes state (melts, boils, freezes, condenses), energy is transferred but the temperature stays constant. This energy breaks or forms the bonds between particles.
🔄 Phase Changes
Solid ⇌ Liquid ⇌ Gas
← Freezing · Condensing → releases energy
← Melting · Evaporating → absorbs energy
During melting or boiling, ALL the energy goes into breaking bonds — none goes into raising temperature. That's why ice melts at 0°C and stays at 0°C until it's all melted!
🔥 Specific Latent Heat: Q = mL
Specific latent heat (L) is the energy needed to change the state of 1 kg of a substance at constant temperature.
⚡ Latent Heat Formula
$$Q = mL$$
$Q$ = energy (J) · $m$ = mass (kg) · $L$ = specific latent heat (J/kg)
| 🧊 Process | 🧪 Material | 📊 L (J/kg) |
|---|---|---|
| Fusion (melting) | Ice → Water | 334,000 |
| Vaporisation | Water → Steam | 2,260,000 |
| Fusion | Lead | 23,000 |
🧊 Example: Energy to melt 0.5 kg of ice at 0°C:
Q = 0.5 × 334,000 = 167,000 J = 167 kJ
Q = 0.5 × 334,000 = 167,000 J = 167 kJ
📈 Heating and Cooling Curves
A heating curve shows temperature vs. time as a substance is heated continuously.
| 📊 Section of curve | 📝 What's happening |
|---|---|
| Rising slope | Temperature increases, solid/liquid/gas gaining KE. Use Q = mcΔT |
| Flat plateau at 0°C (for ice) | Melting — energy breaks bonds, T stays at 0°C. Use Q = mL |
| Rising slope again | Liquid water heats up. Use Q = mcΔT with liquid SHC |
| Flat plateau at 100°C | Boiling — energy breaks bonds to form steam. Use Q = mL |
| Rising slope again | Steam heats up (superheated steam) |
Ready to test yourself? Click the Quiz tab above to answer questions on this topic!
🧊 Latent Heat Calculator (Q = mL)