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What happens to energy and temperature when a substance changes state?

Name the phase changes, interpret a heating curve, and explain why temperature stays constant during a change of state (MA STE supporting content, energy and changes of state).

A standard-level answer on phase changes and heating curves for Massachusetts high school chemistry: naming the six phase changes, reading the flat and sloping sections of a heating curve, and explaining why temperature is constant during melting and boiling, grounded in the framework's energy and matter content.

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

What this topic is asking

This page connects the states of matter to energy. A Massachusetts high school chemistry course expects you to name the changes of state, interpret a heating curve (a graph of temperature against energy added), and explain the key counterintuitive fact: while a substance is melting or boiling, its temperature does not change even though energy is still being added.

The six phase changes

  • Melting (fusion): solid to liquid. Freezing: liquid to solid.
  • Vaporisation (boiling or evaporation): liquid to gas. Condensation: gas to liquid.
  • Sublimation: solid straight to gas (dry ice, carbon dioxide, does this). Deposition: gas straight to solid (frost forming).

The three changes that move toward the gas state pull particles apart and so absorb energy (they are endothermic). The three that move toward the solid state let particles come together and so release energy (they are exothermic).

Reading a heating curve

A heating curve plots temperature on the vertical axis against energy added (or time, at a steady heating rate) on the horizontal axis. It has two kinds of section:

  • Sloping sections are where a single state is being heated. The temperature rises because the added energy increases the particles' kinetic energy, the link to states of matter and kinetic molecular theory.
  • Flat (plateau) sections are where a phase change happens. The temperature holds steady even though energy is still being added.

For water heated from ice, the curve rises (ice warming), flattens at the melting point, rises again (liquid warming), flattens at the boiling point, then rises (steam warming).

Why temperature is constant during a phase change

This is the idea examiners test most. While ice melts, every joule of energy is used to loosen the rigid bonds holding the particles in place, not to make them move faster. Only once the solid has fully melted does further energy start raising the temperature again. The same happens at the boiling point: energy goes into separating the particles into a gas. The stronger the forces between particles, the more energy each plateau needs, which ties back to intermolecular forces and physical properties.

Try this

Q1. Name the phase change when steam turns to liquid water on a cold mirror. [1]

  • Cue. Condensation (gas to liquid), which releases energy.

Q2. Why does a substance with stronger forces between its particles have a higher boiling point? [1]

  • Cue. More energy is needed to separate the particles into a gas, so a higher temperature is required to boil it.

Exam-style practice questions

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

MA Chemistry (style)3 marksA heating curve for water has a flat region at 0 ∘C0\,^{\circ}\text{C} and another at 100 ∘C100\,^{\circ}\text{C}. (a) Name the change at each flat region. (b) Explain why the temperature does not rise there. (c) State what the sloping regions represent.
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A 3-point heating-curve item.

(a) 1 point: the flat region at 0 ∘C0\,^{\circ}\text{C} is melting (fusion); the flat region at 100 ∘C100\,^{\circ}\text{C} is boiling (vaporisation).
(b) 1 point: during a change of state the added energy goes into overcoming the forces between particles, not into speeding them up, so the temperature stays constant.
(c) 1 point: the sloping regions show a single state being heated, where the temperature rises as the particles gain kinetic energy. Markers reward distinguishing energy that breaks forces from energy that raises temperature.

MA Chemistry (style)2 marks(a) Name the phase change from a gas directly to a solid. (b) State whether energy is absorbed or released during freezing.
Show worked answer →

A 2-point phase-change item.

(a) 1 point: deposition (gas to solid directly, the reverse of sublimation).
(b) 1 point: freezing releases energy, because particles move closer together and form stronger attractions, giving out energy to the surroundings (it is exothermic). Markers reward naming deposition and correctly stating that freezing is exothermic.

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