Melting, Boiling, Latent Heat and Evaporation
Changes of state absorb or release energy without changing the temperature. That single fact explains heating curves, latent heat and why evaporation cools you down.
1. Changes of state and energy
| Change | Direction | Energy |
|---|---|---|
| Melting | solid → liquid | absorbed |
| Boiling / evaporating | liquid → gas | absorbed |
| Condensing | gas → liquid | released |
| Freezing | liquid → solid | released |
During a change of state the temperature stays CONSTANT.
Why: the energy supplied is used to break the forces between particles (increasing their potential energy) rather than to make them move faster. Since temperature measures average kinetic energy, and the kinetic energy isn’t changing, the temperature doesn’t change.
This is the explanation the mark scheme wants: energy goes into breaking bonds/forces between particles, not into increasing kinetic energy. Uncertainty about what happens at the melting point in terms of particles and energy was recorded.
2. Heating curves
A graph of temperature against time for a substance heated steadily shows sloping sections and flat plateaus.
- Sloping — the substance is warming; kinetic energy and temperature rise
- Flat (plateau) — a change of state is happening; energy breaks bonds, temperature is constant
Reading a heating curve:
The first plateau is at the melting point; the second is at the boiling point.
Cooling curves are the mirror image — flat sections where the substance freezes or condenses, releasing energy.
3. Latent heat
Latent heat is the energy absorbed or released during a change of state, at constant temperature.
“Latent” means hidden — the energy goes in but the thermometer doesn’t move.
Specific latent heat of FUSION (L_f) — the energy needed to change 1 kg of a substance from solid to liquid at its melting point. Specific latent heat of VAPORISATION (L_v) — the energy needed to change 1 kg from liquid to gas at its boiling point.
Unit: J/kg.
The equation: E = m L energy (J) = mass (kg) × specific latent heat (J/kg)
Example: melting 0.5 kg of ice, with L_f = 334 000 J/kg.
- E = 0.5 × 334 000 = 167 000 J
There is no ΔT in this equation — that is the whole point. Confusing specific latent heat with specific heat capacity was a recorded error; E = mcΔθ involves a temperature change, while E = mL happens at constant temperature.
L_v is much larger than L_f for the same substance (for water, 2 260 000 J/kg vs 334 000 J/kg), because all the forces between particles must be broken to make a gas, not merely loosened.
Multi-stage problems: to take ice at −10 °C to steam, you need E = mcΔθ for each warming stage and E = mL for each change of state, then add them.
4. Evaporation vs boiling
Both turn liquid into gas, but they are different processes.
| Evaporation | Boiling | |
|---|---|---|
| Where | at the surface only | throughout the liquid |
| Temperature | at any temperature | at a fixed temperature (the boiling point) |
| Speed | slow | rapid |
| Bubbles | none | bubbles form throughout |
Evaporation happens at ANY temperature and only at the SURFACE. Two recorded errors here: thinking evaporation happens at a fixed point, and thinking it is impossible at room temperature. A puddle dries on a cold day — that is evaporation.
How evaporation works:
The fastest-moving particles at the surface have enough energy to escape the forces holding them in the liquid.
Why evaporation causes cooling:
Because the fastest particles leave, the average kinetic energy of those remaining falls — and since temperature measures average kinetic energy, the liquid cools.
This is why sweating cools you, and why you feel cold stepping out of a swimming pool.
Factors that increase the rate of evaporation:
- Higher temperature — more particles have enough energy to escape
- Larger surface area — more particles at the surface
- Moving air / draught — removes escaped particles so fewer return
- Lower humidity
A container with a larger surface area cools faster by evaporation — the answer to “which container cools more quickly”, a recorded point of confusion. (Surface area also increases the rate of thermal transfer generally.)
5. Melting and boiling points
The melting point is the temperature at which a solid becomes a liquid; the boiling point is where a liquid becomes a gas. Pure substances have sharp, fixed values.
For water: melting point 0 °C, boiling point 100 °C (at normal atmospheric pressure).
Impurities change these values:
Adding salt LOWERS the melting point of ice.
This is why salt is spread on icy roads — the ice melts at a temperature below 0 °C, so it turns to water when it otherwise wouldn’t.
Salt does not “make ice melt faster” in itself — it lowers the melting point. A recorded error framed it as speed; the correct physics is that the melting point drops.
Pressure also affects boiling point: lower pressure (at altitude) means a lower boiling point, which is why water boils below 100 °C on a mountain.
6. Mistakes that cost marks
Saying the temperature rises during a change of state.
Explaining the plateau without mentioning bonds/forces between particles.
Confusing E = mL with E = mcΔθ.
Putting a temperature change into the latent heat equation.
Saying evaporation happens at a fixed temperature.
Saying evaporation can’t happen at room temperature.
Saying evaporation happens throughout the liquid.
Not explaining cooling via the loss of the fastest particles.
Saying salt makes ice melt faster rather than lowering the melting point.
Omitting units — J/kg for specific latent heat.
Frequently asked questions
Why doesn’t the temperature change during melting? The energy breaks the forces between particles rather than increasing their kinetic energy.
What is latent heat? The energy absorbed or released during a change of state at constant temperature.
What is specific latent heat of fusion? The energy to change 1 kg of solid to liquid at its melting point.
What is the equation? E = m L, with no temperature term.
Why is latent heat of vaporisation larger? All the inter-particle forces must be broken to form a gas.
What is the difference between evaporation and boiling? Evaporation happens at the surface at any temperature; boiling happens throughout at a fixed temperature.
Why does evaporation cause cooling? The fastest particles escape, lowering the average kinetic energy of those left.
What increases the rate of evaporation? Higher temperature, larger surface area, moving air, lower humidity.
Why is salt put on icy roads? It lowers the melting point of ice.
Why does water boil below 100 °C up a mountain? Lower atmospheric pressure lowers the boiling point.
Quick revision checklist
- I know which changes of state absorb and which release energy
- I know temperature is constant during a change of state
- I can explain why, in terms of bonds and kinetic energy
- I can read a heating curve and identify both plateaus
- I can define latent heat and both specific latent heats
- I know E = mL has no ΔT
- I can distinguish it from E = mcΔθ
- I know why L_v > L_f
- I can combine heating and change-of-state stages
- I know the four differences between evaporation and boiling
- I can explain evaporative cooling
- I can list factors affecting the rate of evaporation
- I know salt lowers the melting point
- I know pressure affects boiling point
These notes cover melting, boiling, latent heat and evaporation in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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