Specific latent heat is the energy needed to change the state of 1 kg of a substance without changing its temperature. The equation Q = mL links energy transferred, mass, and latent heat. Temperature stays constant during a state change because energy breaks or forms intermolecular bonds rather than raising particle kinetic energy.
What is latent heat and why is it "latent"?
The word latent means hidden. When a substance changes state — melting, freezing, boiling, or condensing — energy is supplied or released, yet the temperature does not change. The energy is "hidden" inside the substance, used to alter the arrangement of particles rather than to increase their average speed. It only becomes apparent again if you measure the energy flow without watching the thermometer.
This is why a saucepan of boiling water stays at 100 °C however hard you heat it: all the extra energy above that point goes into latent heat of vaporisation, breaking the intermolecular bonds in the liquid rather than raising the temperature further.
What is the difference between latent heat of fusion and latent heat of vaporisation?
There are two specific latent heat values for any substance:
| Type | State change | Typical size |
|---|---|---|
| Specific latent heat of fusion (Lf) | Solid ↔ liquid (melting or freezing) | Smaller — only partial disorder of bonds |
| Specific latent heat of vaporisation (Lv) | Liquid ↔ gas (boiling or condensing) | Much larger — all intermolecular bonds must be broken |
For water: Lf ≈ 334,000 J/kg (334 kJ/kg); Lv ≈ 2,260,000 J/kg (2,260 kJ/kg). The vaporisation value is nearly seven times larger because converting liquid water to steam requires completely breaking the intermolecular forces that hold the molecules together, whereas melting ice only partially disrupts them.
What is the specific latent heat equation?
$$Q = mL$$
Where:
- Q = energy transferred (joules, J)
- m = mass (kilograms, kg)
- L = specific latent heat (J/kg)
Rearranged to find mass or specific latent heat:
$$m = \frac{Q}{L} \qquad L = \frac{Q}{m}$$
There is no temperature change term in this equation — that is the whole point. If a temperature change is also occurring (e.g. heating ice from −10 °C to 0 °C before it melts), you must use $\Delta Q = mc\Delta\theta$ for that section separately.
How do you calculate energy for a state change — worked example?
Worked example 1 — melting ice:
How much energy is needed to melt 500 g of ice at 0 °C? (Specific latent heat of fusion of water = 334,000 J/kg.)
- Convert mass: m = 500 g = 0.500 kg
- Write the equation: Q = mL
- Substitute: Q = 0.500 × 334,000
- Calculate: Q = 167,000 J (167 kJ)
Worked example 2 — condensing steam:
A 200 g sample of steam condenses at 100 °C. How much energy is released? (Lv = 2,260,000 J/kg.)
- Convert mass: m = 0.200 kg
- Q = mL = 0.200 × 2,260,000
- Q = 452,000 J (452 kJ)
This large energy release explains why a steam burn is far more severe than a boiling-water burn of the same mass — steam releases its latent heat of vaporisation as it condenses on skin, in addition to the heat already stored at 100 °C.
Why does temperature stay constant during a state change?
Temperature is a measure of the average kinetic energy of the particles in a substance. During a state change, the supplied energy is used entirely to overcome intermolecular forces — breaking bonds (during melting or boiling) or forming them (during freezing or condensation). The average particle speed — and therefore temperature — does not increase.
On a heating-curve graph, this produces flat horizontal sections at the melting point and boiling point. The length of the flat section indicates how much latent heat is required: a longer flat section means a larger specific latent heat.
How do you measure specific latent heat of ice in the laboratory?
This is a common required-practical approach:
- Set up a funnel of crushed ice with a heater inserted, collecting meltwater in a beaker on a balance. Also run a control funnel (no heater) to measure how much ice melts from the surroundings alone.
- Record the mass of water collected in both funnels over the same measured time.
- Subtract the control mass from the heater funnel mass to find the mass melted by the heater alone (m).
- Calculate the energy supplied: Q = power × time = (voltage × current) × time, or read a joulemeter directly.
- Calculate L = Q ÷ m.
The control funnel is essential: without it, energy from the surroundings inflates the mass figure and gives a specific latent heat value that is too low.
Frequently asked questions
What is specific latent heat in GCSE physics?
Specific latent heat is the energy required to change the state of 1 kg of a substance at constant temperature, measured in joules per kilogram (J/kg). It is calculated using Q = mL. There are two types: latent heat of fusion (for melting and freezing) and latent heat of vaporisation (for boiling and condensing). Temperature does not change during the state change — all the energy alters the bonding between particles.
Why is the specific latent heat of vaporisation much larger than the latent heat of fusion?
Boiling converts a liquid to a gas by completely separating the particles from one another, which requires breaking all the intermolecular bonds that hold the liquid together. Melting a solid to a liquid only partially disrupts those bonds — the particles become mobile but remain in contact. Because vaporisation demands far more bond-breaking, it requires far more energy per kilogram. For water, the difference is roughly sevenfold: 334 kJ/kg to melt versus 2,260 kJ/kg to boil.
How do you rearrange Q = mL to find mass or specific latent heat?
To find mass, rearrange to m = Q ÷ L: divide the energy supplied (in joules) by the specific latent heat (in J/kg). To find the specific latent heat, rearrange to L = Q ÷ m: divide the energy by the mass in kilograms. Both rearrangements follow the same algebraic steps — identify the unknown, divide both sides of Q = mL by the other quantity.
How does specific latent heat differ from specific heat capacity?
Specific heat capacity (c, in J/kg°C) describes energy needed to raise the temperature of 1 kg of a substance by 1 °C — it applies when the substance stays in the same state. Specific latent heat (L, in J/kg) describes energy needed to change the state of 1 kg at constant temperature. On a heating curve, specific heat capacity governs the sloped sections and specific latent heat governs the flat (plateau) sections.
For Socratic GCSE physics with Professor Newton — predict whether a state-change calculation needs Q = mL or Q = mcΔθ before writing any numbers, then check your particle-model reasoning — visit aitutors.me.