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Why does the temperature stay constant while a substance melts or boils, and how do we calculate the energy involved?

Specific latent heat: the energy needed to change the state of a material without changing its temperature, the relationship linking energy, mass and specific latent heat, and the difference between latent heat of fusion and of vaporisation.

An SQA National 5 Physics answer on specific latent heat, covering why temperature stays constant during a change of state, the relationship E equals m times L, the difference between the specific latent heat of fusion and of vaporisation, and how to use it in melting and boiling problems.

Generated by Claude Opus 4.810 min answer

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  1. What this key area is asking
  2. Why temperature stays constant
  3. The specific latent heat relationship
  4. Fusion and vaporisation compared
  5. Combining heat and latent heat
  6. Try this

What this key area is asking

The SQA wants you to explain why temperature stays constant during a change of state, use the relationship E=mLE = mL to find the energy involved, and know the difference between the specific latent heat of fusion (melting) and of vaporisation (boiling).

Why temperature stays constant

A heating graph (temperature against time) shows this clearly: the temperature rises, then flattens out at the melting point while the solid melts, rises again, then flattens once more at the boiling point while the liquid boils. The flat sections are where latent heat is being absorbed.

The specific latent heat relationship

Fusion and vaporisation compared

The latent heat of vaporisation is usually much larger than the latent heat of fusion. For water, fusion is 3.34×105 J kg−13.34 \times 10^5 \text{ J kg}^{-1} but vaporisation is 2.26×106 J kg−12.26 \times 10^6 \text{ J kg}^{-1}, nearly seven times as much. This is because boiling separates the particles completely into a gas, which needs far more energy than melting, which only loosens them from a fixed pattern into a liquid. It also explains why steam at 100 ∘C100\,{}^{\circ}\text{C} scalds far worse than water at 100 ∘C100\,{}^{\circ}\text{C}: as the steam condenses on the skin it releases all of that large latent heat.

Combining heat and latent heat

Some problems involve both warming and a change of state, for example heating ice to its melting point, melting it, then warming the water. Each stage is calculated separately: use E=mcΔTE = mc\Delta T for the stages where the temperature changes and E=mLE = mL for the stages where the state changes, then add the energies together.

Try this

Q1. State what is meant by the specific latent heat of fusion of a material. [1 mark]

  • Cue. The energy needed to melt 1 kg1 \text{ kg} of it without a change in temperature.

Q2. Calculate the energy needed to melt 0.25 kg0.25 \text{ kg} of ice (L=3.34×105 J kg−1L = 3.34 \times 10^5 \text{ J kg}^{-1}). [2 marks]

  • Cue. E=mL=0.25×3.34×105=8.35×104 JE = mL = 0.25 \times 3.34 \times 10^5 = 8.35 \times 10^4 \text{ J}.

Q3. State why the temperature does not change while a substance is boiling. [1 mark]

  • Cue. The energy is used to change the state (break bonds), not to raise the temperature.

Exam-style practice questions

Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

SQA N5 style3 marksCalculate the energy needed to melt 0.40 kg of ice at 0 degrees Celsius. The specific latent heat of fusion of ice is 3.34 times 10 to the power 5 J per kg.
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Use the relationship linking energy, mass and specific latent heat.

Relationship: E=mLE = mL.

Substitution: E=0.40×3.34×105=1.336×105 JE = 0.40 \times 3.34 \times 10^5 = 1.336 \times 10^5 \text{ J}.

Markers reward selecting E=mLE = mL, correct substitution, and a final answer in joules (J\text{J}), about 1.3×105 J1.3 \times 10^5 \text{ J}. No temperature change is involved because the ice stays at 0 ∘C0\,{}^{\circ}\text{C} while it melts.

SQA N5 style4 marksExplain why the temperature of a beaker of boiling water stays at 100 degrees Celsius even though heat is still being supplied.
Show worked answer →

While the water is boiling, the energy supplied is being used to change the state of the water from liquid to gas, not to raise its temperature.

This energy, the latent heat of vaporisation, goes into breaking the bonds between the water molecules so they can escape as a gas, rather than making the molecules move faster (which is what raises temperature).

So the temperature stays constant at the boiling point (100 ∘C100\,{}^{\circ}\text{C}) until all the water has turned to steam. Markers reward stating that the energy changes the state, links to breaking bonds rather than raising temperature, and that the temperature is constant during the change.

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