3.2.4Particle Model of Matter
Specific Latent Heat — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions
5 min read 8 parts Lesson 9 of 12
Part 1 of 6Definition and equation
Specific latent heat
- Specific latent heat The amount of energy needed to change the state of 1 kg of a substance with no change in temperature.
Equation
- Equation E = m × L, where E is energy in joules (J), m is mass in kilograms (kg) and L is specific latent heat in J/kg.
Two values per substance
- Two values per substance Specific latent heat of fusion applies to melting or freezing;
- specific latent heat of vaporisation applies to boiling or condensing.
- For waterfusion about 334 000 J/kg and vaporisation about 2 260 000 J/kg.
Part 2 of 6Worked calculation
1
Question: how much energy is needed to boil away 0.50 kg of water at 100 °C? L = 2 260 000 J/kg.
2
Write the equation E = m × L.
3
Substitute: E = 0.50 × 2 260 000.
4
Answer: E = 1 130 000 J, or 1.13 MJ. To find a mass or L, rearrange to m = E ÷ L or L = E ÷ m.
Part 3 of 6More practice calculations
| Question | Working | Answer |
|---|---|---|
| Energy to melt 2.0 kg of ice already at 0 °C (L = 334 000 J/kg) | E = 2.0 × 334 000 | 668 000 J |
| Mass of water boiled by 452 000 J (L = 2 260 000 J/kg) | m = 452 000 ÷ 2 260 000 | 0.20 kg |
| A 0.25 kg sample of a metal needs 62 500 J to melt at its melting point. Find L. | L = 62 500 ÷ 0.25 | 250 000 J/kg |
| Energy released when 0.10 kg of steam condenses to water at 100 °C | E = 0.10 × 2 260 000 | 226 000 J |
Part 4 of 6Heating ice to steam: two stages
1Warming
- Warming Use ΔE = m × c × Δθ while the temperature changes within one state.
2Changing state
- Changing state Use E = m × L while the temperature is constant at the melting or boiling point.
3Add the stages
- Add the stages For a full journey, such as ice at −10 °C to steam, calculate each stage separately and add the energies together.
Part 5 of 6Measuring L for ice or water
1
Place a heater in a beaker containing a known mass of crushed ice (or water already at its boiling point), connected to a joulemeter or to an ammeter and voltmeter.
2
Switch on and record the electrical energy transferred, E = V × I × t.
3
For vaporisation, place the beaker on a balance and measure the mass of water boiled off.
4
Calculate L = E ÷ m. Insulate the container and repeat to reduce heat losses and improve accuracy.
Part 6 of 6Comparing the two quantities
| Feature | Fusion | Vaporisation |
|---|---|---|
| Change | Solid to liquid | Liquid to gas |
| Typical size for water | Smaller | About seven times larger |
| Why | Particles stay close, only the lattice is loosened | Particles must be separated far apart |
| Equation | E = m × Lf | E = m × Lv |
Key terms
| Term | Meaning |
|---|---|
| Specific latent heat | Energy per kilogram needed to change state without a temperature change. |
| Joule | The unit of energy. |
| Joulemeter | An instrument that records the electrical energy supplied. |
| Fusion | Melting or freezing. |
| Vaporisation | Boiling or condensing. |
| Specific heat capacity | Energy needed to raise the temperature of 1 kg of a substance by 1 °C. |
Avoid these mistakes
1
Using a temperature change in E = m × L — there is no temperature term.
2
Using grams rather than kilograms for the mass.
3
Using the latent heat of fusion for boiling problems, or vice versa.
4
Forgetting that the practical result is usually too high because of energy lost to the surroundings.
5
Using E = m × L for a stage where the temperature is changing instead of m × c × Δθ.
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