Heating Curve

Summary: A graph of temperature against time as a substance is heated at a steady rate, showing sloping sections where one state warms up and horizontal plateaus at the melting and boiling points where changes of state occur. Tags: igcse chemistry Created: 2026-07-17

A heating curve is a graph of temperature (y-axis) against time (x-axis) plotted while a substance is heated at a constant rate, typically taking it from a solid, through the liquid state, to a gas. The curve consists of alternating sloping sections, where the substance exists in a single state and its temperature rises steadily, and horizontal plateaus, where a change of state (melting or boiling) is taking place and the temperature remains constant despite continued heating. The plateaus occur because the energy being supplied is used to overcome the forces of attraction between particles — breaking down the solid lattice at the melting point, or separating the particles completely at the boiling point — rather than to increase the particles’ kinetic energy, which is what would raise the temperature. Heating curves therefore provide a direct experimental way to measure a substance’s melting point and boiling point, which are simply the temperatures of the two flat sections. For a pure substance these plateaus are sharp and perfectly horizontal, whereas an impure substance melts and boils over a range of temperatures, so heating curves are also used as a test of purity. The whole shape of the curve is explained by the Kinetic Particle Theory, and running the process in reverse produces a Cooling Curve.

Shape of a Heating Curve

A typical heating curve for a pure substance heated from below its melting point to above its boiling point has five regions:

Sloping sections = one state present, energy raises the kinetic energy of the particles, so the temperature rises. Horizontal plateaus = two states present together, energy overcomes interparticle forces (increasing the particles’ potential energy), so the temperature stays constant.

Reading Melting and Boiling Points from the Curve

• The melting point is the temperature of the first plateau — the constant temperature at which solid and liquid coexist while the solid melts.
• The boiling point is the temperature of the second plateau — the constant temperature at which liquid and gas coexist while the liquid boils.
• To read them, draw a horizontal line from each flat section across to the temperature axis and read off the value.

For water at standard atmospheric pressure, the plateaus appear at 0 °C (melting) and 100 °C (boiling).

The length of each plateau reflects how much energy the change of state requires at the given heating rate; the boiling plateau is usually longer than the melting plateau because completely separating the particles requires more energy than merely breaking up the lattice.

Pure vs Impure Substances

• A pure substance has a sharp, fixed melting point and boiling point: the plateaus on its heating curve are perfectly horizontal and occur at precise, characteristic temperatures. These values can be checked against data tables to identify the substance.
• An impure substance (a mixture) melts and boils over a range of temperatures: instead of flat plateaus, the curve shows gently sloping “shoulders”. Impurities lower the melting point and raise the boiling point of a substance.
• This is why measuring melting and boiling points is a standard test for purity — for example, pure water boils at exactly 100 °C, while salty water boils above 100 °C and freezes below 0 °C.

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025), Cambridge Assessment International Education

Common Misconceptions