LATENT HEAT is the energy required to change the state of a substance WITHOUT changing its temperature.
When a substance changes state:
Energy is supplied BUT temperature stays constant.
The energy goes into POTENTIAL ENERGY — breaking or forming intermolecular bonds.
Kinetic energy of particles stays the same → temperature stays the same.
'Latent' means 'hidden' — the energy is 'hidden' because it doesn't show up as a temperature change.
There are TWO types:
SPECIFIC LATENT HEAT OF FUSION (Lf): energy to change 1 kg of solid → liquid (melting) or liquid → solid (freezing).
SPECIFIC LATENT HEAT OF VAPORISATION (Lv): energy to change 1 kg of liquid → gas (boiling) or gas → liquid (condensing).
Lv > Lf for most substances — it takes more energy to fully separate particles (boiling) than to just disrupt the lattice (melting).
The Specific Latent Heat Equation
EQUATION:
E = m × L
E = energy for the change of state (J)
m = mass (kg)
L = specific latent heat (J/kg)
Rearranging:
m = E ÷ L
L = E ÷ m
Some values:
Water — latent heat of fusion: 334,000 J/kg (334 kJ/kg)
Water — latent heat of vaporisation: 2,260,000 J/kg (2260 kJ/kg)
Note: vaporisation needs ~7× more energy than fusion for water.
EXAMPLE 1 — melting:
Energy to melt 0.5 kg of ice (Lf = 334,000 J/kg):
E = 0.5 × 334,000 = 167,000 J
EXAMPLE 2 — boiling:
Energy to boil away 2 kg of water at 100°C (Lv = 2,260,000 J/kg):
E = 2 × 2,260,000 = 4,520,000 J = 4.52 MJ
Latent Heat in Context
WHY STEAM CAUSES WORSE BURNS THAN BOILING WATER:
Boiling water (100°C) and steam (100°C) are at the same temperature.
But when steam CONDENSES on skin, it releases the latent heat of vaporisation (2,260,000 J/kg) IN ADDITION to the heat from cooling.
Steam releases much more energy per kg than liquid water at the same temperature.
SWEATING AND EVAPORATIVE COOLING:
When sweat evaporates, it absorbs latent heat of vaporisation from the skin.
This cools the body — the energy taken from the skin during evaporation lowers skin temperature.
High latent heat of vaporisation of water makes this very effective.
FREEZING PONDS:
Water releases latent heat of fusion when it freezes → water surrounding fish slowly releases energy → pond cools slowly.
This is why ponds freeze from the surface DOWN — the surface loses heat to the cold air above.
HEATING CURVE — FLAT SECTIONS:
Flat at 0°C during melting (fusion). Flat at 100°C during boiling (vaporisation).
Slopes in between = temperature rising using ΔE = mcΔθ.
⚠️ Common Mistake
During a change of state, temperature does NOT change — the flat sections on a heating curve. E = mL has NO temperature change term — unlike ΔE = mcΔθ. Students often confuse the two equations. If temperature is changing, use ΔE = mcΔθ. If state is changing (temperature constant), use E = mL.
📐 Variables
EEnergy for change of state (E) is measured in joules (J)
mMass (m) is measured in kilograms (kg)
LSpecific latent heat (L) is measured in J/kg (J/kg)
📐 Key Equations
E = m × L
📌 Key Note
E = mL. No temperature change during state change — energy goes into PE (breaking bonds). Fusion: solid ↔ liquid. Vaporisation: liquid ↔ gas. Lv > Lf (more energy to boil than melt). Water Lf = 334 kJ/kg; Lv = 2260 kJ/kg. Steam burns worse than boiling water — releases latent heat on condensing.
🎯 Matching Activity — Latent Heat Concepts
Match each term to its correct description. — drag the symbols on the right to match the component names on the left.
Latent heat of fusion
Drop here
Latent heat of vaporisation
Drop here
E = mL
Drop here
Why steam burns worse
Drop here
Flat section on heating curve
Drop here
Energy to change 1 kg liquid ↔ gas with NO temperature change — larger than fusion
Energy to change 1 kg solid ↔ liquid with NO temperature change
Steam releases latent heat of vaporisation on condensing — extra energy on top of cooling
Equation for energy in a change of state — no Δθ term
Temperature constant during change of state — energy increasing PE, not KE
⚽ FIFA Worked Examples
Latent Heat — Melting
Calculate the energy needed to melt 2 kg of ice. (Lf = 334,000 J/kg)
F
E = m × L
I
m = 2 kg, L = 334,000 J/kg
F
E = 2 × 334,000
A
E = 668,000 J (668 kJ)
🎯 Test Yourself
Question 1 of 2
1. How much energy is released when 0.5 kg of steam at 100°C condenses to water at 100°C? (Lv = 2,260,000 J/kg)
2. Why does sweating cool the body?
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