Energy and particles · GCSE Physics
Specific latent heat
Teacher-written GCSE Physics revision on specific latent heat: E = mL for melting and boiling at constant temperature, fusion versus vaporisation, and why heating curves go flat.
Latent heat changes state, not temperature. E = mL. Fusion is solid to liquid; vaporisation is liquid to gas. Temperature stays constant while the change happens.
The important bits
What you need to know
- 1
Specific latent heat L is the energy to change the state of 1 kg of a substance without changing its temperature. Units: J/kg.
- 2
E = mL, with E in joules and m in kilograms. Use L_f for fusion (melting or freezing) and L_v for vaporisation (boiling or condensing).
- 3
During melting or boiling, energy goes into changing the potential store of the particles as intermolecular forces are broken, not into raising kinetic energy, so temperature is constant.
- 4
L_v is much larger than L_f for water: boiling has to separate particles completely, melting only has to loosen the solid structure.
- 5
Mass is conserved at a change of state. 0.10 kg of ice becomes 0.10 kg of water. Density and volume change; mass does not.
- 6
A heating curve of ice to steam has two flat sections: melting at 0 °C and boiling at 100 °C. Sloped sections use ΔE = mcΔθ.
- 7
To heat ice at −10 °C to steam you must split the journey: warm the ice, melt it, warm the water, then boil it. Add the energies. One formula will not do.
- 8
Condensing and freezing release energy: the same mL comes out of the store as particles form stronger attractions.
Quotations worth analysing
Short evidence. Real method.
“E = mL”
Use this only while state is changing. If temperature is also changing, you need mcΔθ as well, in a separate line.
“Temperature does not change during a change of state.”
Students mark the flat bits as “no energy transferred”. The opposite is true: energy is still arriving, but it is not raising temperature.
“Specific latent heat of vaporisation is the energy to change 1 kg from liquid to gas at constant temperature.”
The phrase “at constant temperature” is part of the definition. Leave it out and the mark scheme may not accept the line.
Go deeper
Temperature and state are different jobs for the same energy
When you heat ice at 0 °C, the temperature does not rise until it has all melted. The energy is still being transferred; it is increasing the potential store as bonds between particles loosen. That energy is mL. After that, temperature rises according to mcΔθ until 100 °C, then another plateau for boiling. A heating curve with two flat sections is this story as a graph. Students often mark the flat bits as “no energy transferred”. The opposite is true. If a question gives L and c, split the problem into chunks: heat solid, melt, heat liquid. Add the energies. Do not try to use one formula for the whole journey from freezer to kettle.
Go deeper
Why vaporisation costs more energy than fusion
Melting a solid loosens a regular structure so particles can flow; they remain close. Boiling has to pull particles so far apart that they become a gas, which takes much more work against attractive forces. For water, L_f is about 3.3 × 10⁵ J/kg and L_v about 2.3 × 10⁶ J/kg — roughly seven times larger. That is why a burn from steam can be worse than a burn from boiling water: condensing steam dumps that extra latent heat onto the skin. In calculations, check you have picked L_f or L_v to match melting or boiling. Using the wrong L is as serious as using the wrong equation.
See the idea in action
A 0.12 kg block of ice at 0 °C melts. L_f for ice = 3.3 × 10⁵ J/kg. Energy to melt = mL = 0.12 × 3.3 × 10⁵ = 39 600 J. The temperature is still 0 °C when it has just become water. To raise that water to 20 °C, ΔE = mcΔθ = 0.12 × 4200 × 20 = 10 080 J. Total energy = 49 680 J, about 50 kJ. Using only mcΔθ from the start would miss the latent heat and understate the energy badly.
Exam technique
Turn knowledge into marks
State whether temperature is changing (use mcΔθ) or state is changing (use mL). Keep mass in kilograms. On a heating curve, the flat sections are latent heat, not “no energy”.
Common mistakes
Do not give these marks away
- 01
Using ΔE = mcΔθ during melting or boiling, when temperature is constant.
- 02
Mixing up L_f and L_v, or leaving mass in grams so E is a thousand times too small.
- 03
Saying particles in a liquid do not move, or that mass changes when ice melts.
Why does temperature stay constant while ice melts, even though it is still being heated?
ANo energy is being transferred
BEnergy is used to break intermolecular forces rather than to raise the particles’ kinetic energy
CThe mass of the ice decreases
DLatent heat only applies to gases
Show the answer
Energy is used to break intermolecular forces rather than to raise the particles’ kinetic energy. The potential store increases as the solid becomes liquid. Kinetic energy, which we measure as temperature, stays the same until melting is complete.
Quick questions
If this is the bit you searched
What is the difference between specific heat capacity and specific latent heat?
Specific heat capacity is energy to change temperature (ΔE = mcΔθ). Specific latent heat is energy to change state at constant temperature (E = mL).
What is the difference between fusion and vaporisation?
Fusion is solid to liquid (or the reverse, freezing). Vaporisation is liquid to gas (or the reverse, condensing). Each has its own L.
Does mass change when water boils?
The mass of water plus steam stays the same. If steam leaves the kettle, the liquid left behind has less mass, but those particles still exist as gas.
How do you read a heating curve?
Sloped sections: temperature is changing, use mcΔθ. Flat sections: state is changing at constant temperature, use mL. Ice melts at 0 °C; water boils at 100 °C at standard pressure.