GCSE Physics: Changes of state and specific latent heat

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1. What is latent heat?

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   The energy needed for a substance to change state.

2. During a change of state, what happens to the temperature of a substance?

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   It stays constant — the energy supplied changes the internal energy, not the temperature.

3. Define specific latent heat.

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   The energy required to change the state of one kilogram of a substance with no change in temperature.

4. What is the equation for the energy needed for a change of state?

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   E = m L (energy = mass × specific latent heat).

5. What is the SI unit of specific latent heat?

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   Joules per kilogram (J/kg).

6. Specific latent heat of ? is for the change of state from solid to liquid.

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   fusion

7. Specific latent heat of ? is for the change of state from liquid to vapour.

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   vaporisation

8. What change of state does specific latent heat of fusion refer to?

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   Solid to liquid (melting).

9. What is the key difference between specific heat capacity and specific latent heat?

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   Specific heat capacity is the energy to raise 1 kg by 1 °C; specific latent heat is the energy to change the state of 1 kg with no temperature change.

10. On a heating graph, what does a flat (horizontal) section indicate?

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    A change of state — energy is being supplied but the temperature stays constant.

11. How much energy is needed to melt 2 kg of ice if L (fusion) is 334 000 J/kg?

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    E = mL = 2 × 334 000 = 668 000 J (668 kJ).