4.2.1Properties of Matter
Specific Latent Heat
5 min read 5 parts Lesson 5 of 11
Section 1 of 5Changes of State
Substances can change between solid, liquid and gas when energy is gained or lost.
Melting
- A solid changes to a liquid at its melting point by gaining energy.
Boiling
- A liquid changes to a gas at its boiling point by gaining energy.
Cooling changes
- Freezing and condensing release energy as particles come closer together.
Constant temperature
- During a change of state the temperature stays constant even though energy is still being transferred.
Section 2 of 5Heating Curve of Water
Following the temperature while ice is heated shows each stage.
1
Ice below 0 °C warms up as the particles vibrate more strongly, so the temperature rises.
2
At 0 °C the ice melts while the temperature stays constant, because the energy is used to separate the particles.
3
When all of the ice has melted the liquid water warms and the temperature rises again.
4
At 100 °C the water boils and the temperature is steady until all the liquid has become steam.
5
Once only steam remains, further heating raises its temperature once more.
Section 3 of 5Latent Heat
The energy used in a change of state is called latent heat.
Latent Heat
Fusion
- Specific latent heat of fusion is the energy to change 1 kg of solid to liquid with no temperature change.
Vaporisation
- Specific latent heat of vaporisation is the energy to change 1 kg of liquid to gas with no temperature change.
Unit
- Specific latent heat is measured in joules per kilogram (J/kg).
Particle view
- The energy increases the potential energy of the particles as they separate, not their kinetic energy.
| Water | Value (J/kg) |
|---|---|
| Specific latent heat of fusion | 3.34 × 10⁵ |
| Specific latent heat of vaporisation | 22.6 × 10⁵ |
Section 4 of 5Worked Example
Question Compare the energy released when 1.0 kg of steam condenses with the energy released when 1.0 kg of water freezes.
- Condensing releases the latent heat of vaporisation: 22.6 × 10⁵ J.
- Freezing releases the latent heat of fusion: 3.34 × 10⁵ J.
- Ratio = 22.6 × 10⁵ ÷ 3.34 × 10⁵ = 6.8.
Answer Condensing steam releases about 6.8 times as much energy, which is why steam burns are so severe.
Section 5 of 5Exam Checklist & Tips
4.2.1 – Key Points for the Exam
- Specific latent heat The energy needed to change the state of 1 kg of a substance without changing its temperature.
- Fusion Change between solid and liquid.
- Vaporisation Change between liquid and gas.
- Melting point The temperature at which a solid becomes a liquid.
Common Mistakes
- Saying the temperature rises during melting or boiling.
- Mixing up fusion (solid to liquid) with vaporisation (liquid to gas).
- Thinking the energy is used to increase the kinetic energy of the particles during a change of state.
- Forgetting that freezing and condensing give energy out.