3.6 Specific latent heat
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The specific latent heat of fusion of water is 334 000 J/kg. Calculate the energy needed to melt 2.0 kg of ice at 0 °C.
Mark scheme
E = m L (1); 2.0 × 334 000 (1); 668 000 J (1)
Model answer
E = m L = 2.0 × 334 000 = 668 000 J.
4 520 000 J of energy boils a mass of water at 100 °C. The specific latent heat of vaporisation of water is 2 260 000 J/kg. Calculate the mass of water.
Mark scheme
m = E / L (1); 4 520 000 / 2 260 000 (1); 2.0 kg (1)
Model answer
m = E / L = 4 520 000 / 2 260 000 = 2.0 kg.
Define the term specific latent heat.
Mark scheme
the energy needed to change the state of 1 kg of a substance (1); with no change in temperature (1)
Model answer
The energy needed to change the state of 1 kg of a substance without changing its temperature.
Explain why the temperature of boiling water stays at 100 °C even though it is still being heated.
Mark scheme
the energy supplied is used to change state / to separate the particles (1); this increases the potential energy of the particles (1); the average kinetic energy of the particles does not change, so the temperature stays constant (1)
Model answer
The energy supplied is used to separate the particles and change the water into steam. This increases the potential energy of the particles but not their kinetic energy, so the temperature stays constant.
Explain why the specific latent heat of vaporisation of water is much larger than the specific latent heat of fusion.
Mark scheme
when boiling the particles must be completely separated / all the forces between particles must be overcome (1); and the gas has to expand against the atmosphere, doing work (1)
Model answer
When water boils, the particles have to be completely separated, which needs much more energy than loosening them in melting. The steam also does work as it expands against the atmosphere.
Which unit is used for specific latent heat?
Choose one answer.
Correct Not quite: the answer is J/kg
Mark scheme
A: J/kg
Model answer
J/kg (A), because L = E / m.
Calculate the energy needed to change 0.10 kg of ice at 0 °C into water at 30 °C. (L = 334 000 J/kg, c = 4200 J/kg °C)
Mark scheme
melting: 0.10 × 334 000 = 33 400 J (1); warming: 0.10 × 4200 × 30 (1); = 12 600 J (1); total = 46 000 J (1)
Model answer
Melting: 0.10 × 334 000 = 33 400 J. Warming: 0.10 × 4200 × 30 = 12 600 J. Total = 46 000 J.
A heater supplies 600 000 J to a pan of boiling water, and 0.26 kg of the water is turned into steam. Calculate the specific latent heat of vaporisation of water.
Mark scheme
L = E ÷ m (1); 600 000 ÷ 0.26 (1); 2.3 × 10⁶ J/kg (1)
Model answer
L = E / m = 600 000 / 0.26 = 2.3 × 10⁶ J/kg.
Explain why steam at 100 °C causes a worse burn than boiling water at 100 °C.
Mark scheme
when steam condenses it releases latent heat (1); this is a large amount of energy for each kilogram (1); in addition to the energy released as the water formed cools down (so much more energy is transferred to the skin) (1)
Model answer
When the steam condenses on the skin, it releases a large amount of latent heat, as well as the energy released when the water cools. So more energy is transferred to the skin than from boiling water.
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