Latent heat · Year 12
In Year 12 you get fluent with: specific latent heat: E = mL; heating and cooling curves.
When a substance melts or boils, the energy supplied breaks the bonds between its particles instead of raising the temperature, so the temperature stays the same: the flat parts of a heating curve. The energy needed to change the state of 1 kg is the specific latent heat, L, and the energy for any mass is E = mL.
Key facts
- Energy to change state: E = mL (L = specific latent heat, J/kg).
- Water: L (fusion) = 334,000 J/kg; L (vaporisation) = 2,260,000 J/kg.
- During melting or boiling the temperature stays constant (flat parts of a heating curve).
Worked examples
Example 1
While water is boiling, where does the energy supplied go?
Answer: into breaking the bonds between particles (latent heat), not raising the temperature
Example 2
A heating curve for ice shows a flat part at 0°C, even though the ice is still being heated. Why?
Answer: the ice is melting: the energy breaks the bonds between the particles instead of raising the temperature (latent heat)
Example 3
Water at 20°C is heated steadily until it has been boiling for 5 minutes. Describe the shape of its temperature–time graph.
Answer: A line rising from 20°C to 100°C, then a flat line at 100°C while it boils.
Example 4
Liquid stearic acid is left to cool. Its temperature stays at 69°C for several minutes. What is happening, and why does the temperature not fall?
Answer: It is freezing: as bonds form between the particles, energy is released, which keeps the temperature at 69°C until it is all solid.
Example 5
How much energy is needed to melt 0.7 kg of ice on a car windscreen at 0°C? (Specific latent heat of fusion of ice = 334,000 J/kg.)
Answer: 233,800 J
Example 6
How much energy is needed to boil away 0.20 kg of water that is already at 100°C? (specific latent heat of vaporisation: 2,260,000 J/kg)
Answer: 452,000 J (452 kJ)
Example 7
How much energy is needed to boil away 1 kg of water already at 100°C? (Specific latent heat of vaporisation of water = 2,260,000 J/kg.) Give your answer in MJ.
Answer: 2.26 MJ