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Why does temperature stay constant while ice melts, and what does a heating curve show?

Phase changes and heating curves: name the phase changes and their energy changes, and interpret a heating or cooling curve including the plateaus.

A focused Virginia SOL Chemistry answer on phase changes under CH.4: the names and energy direction of melting, freezing, vaporization, condensation and sublimation, and how to read a heating curve, including why temperature stays constant during a phase change.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this topic is asking
  2. The phase changes
  3. Why temperature is constant during a phase change
  4. Reading a heating curve
  5. Try this

What this topic is asking

Continuing standard CH.4, Virginia expects you to name the phase changes and state whether each absorbs or releases energy, and to interpret a heating or cooling curve, especially the flat plateaus where a phase change happens. The key idea is that during a phase change the temperature stays constant because energy is used to change the arrangement of particles rather than their speed.

The phase changes

The direction of energy flow follows the change in order. Going to a less ordered state (more particle freedom) requires energy input, so melting, vaporization and sublimation are endothermic (absorb energy). Going to a more ordered state releases energy, so freezing, condensation and deposition are exothermic (release energy).

Why temperature is constant during a phase change

So while ice melts at 0 ∘C0\,^{\circ}\text{C}, all the added heat is used to pull the water molecules out of their fixed lattice; the temperature does not rise until every bit of ice has melted. The same holds at the boiling point. This is why a phase change appears as a horizontal line on a temperature-versus-time graph.

Reading a heating curve

A heating curve plots temperature against time (or heat added) as a substance is heated steadily from solid to gas. It has alternating sloped and flat regions:

  • Sloped sections show a single phase being warmed: the particles gain kinetic energy and the temperature rises. The solid warms, then (after melting) the liquid warms, then (after boiling) the gas warms.
  • Flat plateaus show phase changes at constant temperature: the first plateau is melting (at the melting point) and the second is boiling (at the boiling point). Both phases coexist along a plateau.

A cooling curve is the mirror image, sloping down with plateaus at the freezing and condensation points.

Try this

Q1. Name the phase change from liquid to gas. [1 point]

  • Cue. Vaporization (boiling or evaporation).

Q2. State whether freezing absorbs or releases energy. [1 point]

  • Cue. Releases energy; the particles slow and form a more ordered solid, so it is exothermic.

Exam-style practice questions

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

SOL (multiple choice)1 marksDuring the melting of a pure solid, the temperature of the sample (A) increases (B) decreases (C) remains constant (D) increases then decreases
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The answer is (C) remains constant.

During a phase change the added energy goes into breaking the attractions between particles, not into increasing their average kinetic energy, so the temperature stays constant until the change is complete. On a heating curve this shows as a flat plateau at the melting point. Once all the solid has melted, the temperature rises again.

The trap is assuming continuous heating always raises temperature; during a phase change the energy changes the state, not the temperature.

SOL (tech-enhanced, ordering)2 marks(a) Name the phase change from a gas directly to a solid. (b) State whether energy is absorbed or released during condensation.
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A 2-point item on phase-change names and energy.

(a) Phase change (1 point): deposition (gas to solid directly).
(b) Condensation (1 point): energy is released, because particles slow and come together as a gas becomes a liquid.

Markers reward the correct name and the correct energy direction. Changes toward a more ordered state (condensation, freezing, deposition) release energy; changes toward a less ordered state (melting, vaporization, sublimation) absorb energy.

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