Cooling Curve
Summary: A graph of temperature against time as a substance cools, showing sloping sections where one state loses heat and horizontal plateaus at the condensation point and freezing point where changes of state release energy. Tags: igcse chemistry Created: 2026-07-17
A cooling curve is a graph of temperature (y-axis) against time (x-axis) plotted while a substance cools at a steady rate, typically taking it from a gas, through the liquid state, to a solid. It is the exact reverse of a heating curve: the temperature falls along the sloping sections, where a single state is losing heat and its particles are slowing down, and remains constant along the horizontal plateaus, where a change of state — condensation or freezing — is taking place. The plateaus occur because, as the particles come together and new forces of attraction form between them, energy is released to the surroundings; this released energy exactly compensates for the heat being lost, so the temperature does not fall until the change of state is complete. The first plateau occurs at the condensation point (equal to the boiling point), where gas turns to liquid, and the second occurs at the freezing point (equal to the melting point), where liquid turns to solid. As with heating curves, a pure substance freezes and condenses at sharp, fixed temperatures giving flat plateaus, while an impure substance changes state over a range. The entire shape of the curve is explained by the Kinetic Particle Theory, and it should be studied alongside the Heating Curve, of which it is the mirror image.
Shape of a Cooling Curve
A typical cooling curve for a pure substance cooled from the gas state to below its freezing point has five regions:
| Region | Shape | State(s) present | What is happening to the particles |
|---|---|---|---|
| 1 | Sloping downwards | Gas | Fast-moving particles lose kinetic energy and slow down; temperature falls |
| 2 | Horizontal (plateau at the condensation point = boiling point) | Gas + liquid | Particles come close together and attractive forces form between them; energy is released, keeping the temperature constant |
| 3 | Sloping downwards | Liquid | Particles slide past each other more and more slowly; temperature falls |
| 4 | Horizontal (plateau at the freezing point = melting point) | Liquid + solid | Particles settle into a regular lattice as strong attractive forces lock them in place; energy is released, keeping the temperature constant |
| 5 | Sloping downwards | Solid | Particles vibrate less and less vigorously in the lattice; temperature falls |
Sloping sections = one state present; heat lost comes from the particles’ kinetic energy, so the temperature falls. Horizontal plateaus = two states present together; the energy released as interparticle forces form replaces the heat being lost, so the temperature stays constant.
The Reverse of the Heating Curve
For a pure substance, the cooling curve is the mirror image of its Heating Curve:
| Heating curve feature | Cooling curve equivalent |
|---|---|
| Temperature rises along sloping sections | Temperature falls along sloping sections |
| First plateau: melting at the melting point (energy absorbed to break the lattice) | Second plateau: freezing at the freezing point (energy released as the lattice forms) |
| Second plateau: boiling at the boiling point (energy absorbed to separate particles) | First plateau: condensation at the condensation point (energy released as particles come together) |
| Energy is taken in from the heater | Energy is given out to the surroundings |
Crucially, the plateaus occur at the same temperatures in both directions: a pure substance melts and freezes at exactly the same temperature, and boils and condenses at exactly the same temperature. For water these are 0 °C and 100 °C respectively.
Reading Values and Testing Purity
- The freezing point is read as the temperature of the lower plateau; the condensation point is the temperature of the upper plateau.
- A pure substance gives sharp, horizontal plateaus at fixed, characteristic temperatures, so a cooling curve (often of a melted solid such as stearic acid re-solidifying) is a common laboratory method for determining freezing/melting points.
- An impure substance freezes over a range of temperatures, so the plateau is replaced by a shallow slope, and the freezing point is lower than that of the pure substance. This is the principle behind spreading salt on icy roads.
Sources
- Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025), Cambridge Assessment International Education
Common Misconceptions
| Misconception | Reality |
|---|---|
| During the flat part of a cooling curve the substance has stopped losing heat. | The substance is still losing heat to the surroundings throughout the plateau. The temperature stays constant because energy released as attractive forces form between particles (during condensation or freezing) balances the heat being lost. |
| Freezing happens at a different temperature from melting. | For a pure substance, freezing and melting occur at exactly the same temperature — the freezing point and melting point are identical. The plateau on the cooling curve appears at the same temperature as the melting plateau on the heating curve. |