Knowledge Organiser: Specific Latent Heat
Key Terms
- Specific latent heat (L): energy per kg for state change at constant temperature
- Fusion (Lf): solid ↔ liquid; for water = 334,000 J/kg
- Vaporisation (Lv): liquid ↔ gas; for water = 2,260,000 J/kg
- Latent: "hidden" — temperature does not change
Key Facts
- Temperature stays CONSTANT during state changes
- Energy goes into potential energy (bond breaking), not kinetic energy
- Lv > Lf because vaporisation fully breaks all bonds
- Steam burns release latent heat AND then cool — more damaging than boiling water
Key Equations
- E = mL (state change at constant temperature)
- E = mcΔT (temperature change within a state)
- L units: J/kg
- E in J, m in kg
Exam Tips
- Use E = mL for flat sections of heating curve
- Use E = mcΔT for sloped sections
- Always state temperature constant during latent heat
- Link bond-breaking to potential energy increase
Common Mistakes
- Saying temperature rises during latent heat: Temperature stays constant during a state change — all the energy goes into breaking bonds, not increasing kinetic energy
- Using the wrong equation: Use E = mL for state changes (flat part of graph); use E = mcΔT for temperature changes (sloped part) — mixing these up loses all marks
- Forgetting mass must be in kg: The equation E = mL requires mass in kg — if given in grams, divide by 1000 first
- Confusing latent heat of fusion and vaporisation: Lf = melting/freezing (solid↔liquid); Lv = boiling/condensing (liquid↔gas) — vaporisation requires much more energy
- Not explaining WHY temperature stays flat: Examiners want to hear that energy breaks intermolecular bonds and increases potential energy — not just "it stays constant"
Memory Aid
- Fusion = Freezing/melting (solid-liquid boundary) — think "F for Floor" (the lower level)
- Vaporisation = Very hot (liquid-gas boundary) — think "V for Very high energy"