Latent Heat — Quick Summary

Mr Toogood's Physics · Thermal physics

AQA 3.6.2.1
Q = ml
Latent heat
l = (IVt−I₂V₂t₂)/(m−m₂)
Finding l experimentally
l_vap ≈ 7 × l_fus
For water
ΔPE only
No ΔKE, no ΔT

What is latent heat?

Temperature against time graph as a solid is heated through melting and boiling, showing flat plateaus during phase changes

Temperature plateaus during a phase change, even though heating continues.

While heating a substance through a phase change, all the energy supplied goes into breaking intermolecular bonds — increasing potential energy, not kinetic energy — so temperature stays constant.

Q = ml
  • Q = heat supplied (J)   m = mass (kg)   l = specific latent heat (J kg⁻¹)
  • Fusion: melting/freezing. Vaporisation: boiling/condensing.
Why vaporisation needs so much more energy: melting only breaks some bonds (a liquid still has fairly strong bonds); vaporising breaks all of them — this is why l_vap ≈ 7 × l_fus for water.

Why ice cools drinks better than 0 °C water

Ice must absorb its latent heat of fusion to melt before its temperature can rise above 0 °C. This extra energy absorption means more heat is drawn from the drink before thermal equilibrium is reached, compared to water already at 0 °C.

Measuring specific latent heat

Apparatus for measuring the specific latent heat of vaporisation of a liquid

Heater vaporises liquid at its boiling point; mass collected is measured over time.

Directly analogous to the continuous flow method for SHC: since the liquid is at its boiling point in both trials, the heat lost to surroundings, E, is identical and cancels on subtraction:

IVt = ml + E     I₂V₂t₂ = m₂l + E
l = (IVt − I₂V₂t₂) / (m − m₂)
No Δθ term here — unlike the SHC continuous flow formula — because temperature doesn't change during a phase change.

Multi-stage problems

Questions combining cooling and freezing (or heating and melting) require calculating each stage separately with Q=mcΔθ and Q=ml, then adding the results. Don't round until the very final answer.

Exam essentials

Key equations

  • Q=ml
  • l=(IVt−I₂V₂t₂)/(m−m₂)

Multi-stage method

  • Split into separate cooling/heating and phase-change stages.
  • Calculate each Q separately, then sum.
  • Keep full precision until the final answer.

Common slips

  • Never use Q=mcΔθ across a phase-change plateau — temperature isn't changing there.
  • Latent heat changes PE of particles, not KE — temperature is unaffected.
  • Check whether the question needs the fusion or vaporisation value of l.