AQA Physics (8463) 4.3.2.3 · Quantitative
Changes of state and specific latent heat
Steam and boiling water are both at 100 °C, yet steam does far more damage. What makes the difference, and how do you put a number on it?
Triple Science · Foundation tier
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Same temperature, worse burn.
A splash of boiling water on your hand hurts. A puff of steam from a kettle spout, at the same 100 °C, can leave a much worse burn.
If you had to guess, is the steam worse because it spreads over more skin, or because it gives the skin more energy?
When a substance changes state, the energy supplied changes its internal energy but not its temperature. The energy needed to change state is called latent heat. The specific latent heat of a substance is the energy needed to change the state of 1 kg of it with no change in temperature. Melting, solid to liquid, takes the specific latent heat of fusion. Boiling, liquid to vapour, takes the specific latent heat of vaporisation. Freezing and condensing give the same energy back. For water, fusion takes 334 000 J/kg and vaporisation 2 260 000 J/kg.
The energy ledger
10 g of boiling water and 10 g of steam each land on skin at 37 °C.
Commit first. Compared with the boiling water, how much energy does the steam give the skin?
Equations · choose by what is changing
E = m L
State changing, temperature constant. Energy for a change of state (J) = mass (kg) × specific latent heat (J/kg).
m = E ÷ L · L = E ÷ m
ΔE = m c Δθ
Temperature changing, state constant. c is the specific heat capacity (J/kg °C).
E = P t
Energy transferred at a steady rate: power (W) × time (s).
On a temperature–time graph, a sloping section means the temperature is changing: ΔE = m c Δθ. A flat section means the state is changing: E = m L. When energy goes in or out at a steady rate, the length of a flat section measures the energy, through E = P t.
Energy from a flat section
60 g of liquid Y cools in a cold room.
Y transfers energy to the room at a steady 30 J every second. A data logger records its temperature.
Command words in this lesson
- Determine
- Get it from the graph: a temperature from a flat section, a time from where it starts and ends.
- Calculate
- Write every line, starting with any conversion. In two stages, one equation per stage, then add.
- Explain
- Link the change of state to where the energy comes from or goes, then to what happens.
- Compare
- Say how both are alike and how they differ, naming both every time.
Key fact
State changing at constant temperature: E = m L. Temperature changing: ΔE = m c Δθ. Specific latent heat is the energy needed to change the state of 1 kg with no change in temperature.
Key note · AQA 4.3.2.3 (8463)
Changes of state and specific latent heat
- 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.
Photograph this card. Tomorrow, cover it and say each line first.
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Not sure whether a question needs m c Δθ, m L, or both?
The ledger assumes all 10 g of steam condenses on the skin and the water then cools to skin temperature, with no energy going anywhere else. Particles are drawn as spheres and bars are to scale. Y and Z are illustrative substances: Y loses energy at a perfectly steady rate, so its sloping sections are drawn straight. Water's values are at normal atmospheric pressure.