Melting
Summary: The endothermic change of state from solid to liquid that occurs at a fixed temperature called the melting point. Tags: igcse chemistry Created: 2026-07-17
Melting is the change of state in which a solid becomes a liquid. It is an endothermic process, meaning that energy must be absorbed from the surroundings for it to occur. As a solid is heated, its particles gain kinetic energy and vibrate more and more vigorously about their fixed positions in the lattice. At the melting point, the particles have gained enough energy to partially overcome the strong forces of attraction holding them in their fixed positions, allowing them to move around and slide past one another as a liquid. While a pure substance is melting, its temperature stays constant at the melting point, because all the energy being supplied is used to weaken the forces between particles rather than to raise the temperature. For a pure substance, the melting point is a fixed, characteristic temperature and is numerically equal to its freezing point.
Melting as a Change of State
Melting is one of the interconversions between the three states of matter described by the kinetic particle theory. It is the reverse of freezing (solidification).
Melting: the change of state from solid to liquid, occurring at the melting point, with energy absorbed from the surroundings (endothermic).
What happens to the particles
| Stage | Particle behaviour |
|---|---|
| Solid below the melting point | Particles are closely packed in a regular lattice, held in fixed positions by strong forces of attraction; they vibrate about fixed positions. |
| Heating the solid | Particles absorb energy and gain kinetic energy; vibrations become larger and more vigorous. |
| At the melting point | Particles have enough energy to partially overcome the forces of attraction holding them in the lattice; the regular structure breaks down. |
| Liquid formed | Particles remain close together but are irregularly arranged and can move past one another. |
Key points about the process:
- Melting is endothermic: energy is transferred from the surroundings (or a heater) to the substance.
- The energy absorbed at the melting point is used to overcome the forces of attraction between particles, not to increase their kinetic energy.
- Because the average kinetic energy of the particles does not change while the forces are being overcome, the temperature remains constant during melting. This produces the flat, horizontal section seen on a heating curve.
- Only the arrangement and movement of the particles change; the particles themselves are unchanged. Melting is a physical change, not a chemical change, and no new substance is formed.
Melting point
The melting point is the temperature at which a solid changes to a liquid at a given pressure. For a pure substance at atmospheric pressure it is a sharp, fixed value.
| Substance | Melting point (°C) |
|---|---|
| Oxygen | −219 |
| Ethanol | −117 |
| Water (ice) | 0 |
| Sodium chloride | 801 |
| Iron | 1538 |
The melting point reflects the strength of the forces between particles: substances with strong forces of attraction between their particles (for example ionic compounds such as sodium chloride, or metals such as iron) have high melting points, whereas substances with weak attractive forces between molecules (such as oxygen or ethanol) have low melting points.
Melting point and freezing point
For a pure substance, the melting point and the freezing point are the same temperature; the two names simply describe the direction of the change:
- Melting (solid → liquid) occurs when energy is absorbed at this temperature.
- Freezing (liquid → solid) occurs when energy is released at this same temperature.
For example, pure water melts at 0 °C and pure water also freezes at 0 °C.
Effect of impurities
The presence of an impurity lowers the melting point of a substance and causes it to melt over a range of temperatures rather than sharply at one temperature. This is why measuring a melting point is used as a test of purity: a pure substance melts sharply at its known, fixed melting point, while an impure sample melts at a lower temperature and over a spread of temperatures. A practical application is spreading salt on icy roads, which lowers the melting point of the ice so that it melts below 0 °C.
Melting on a heating curve
If a solid is heated at a steady rate and its temperature is plotted against time, the graph shows:
- A rising line as the solid warms up (particles gain kinetic energy, temperature increases).
- A horizontal (flat) section at the melting point, while the solid and liquid exist together and the energy supplied is used to overcome the forces between particles.
- A rising line again once all the solid has melted and the liquid warms up.
The temperature of the flat section is the melting point of the substance.
Melting Point
The melting point is the specific temperature at which a solid melts — that is, the temperature at which the particles gain enough energy to overcome the forces holding them in fixed positions and the regular lattice structure breaks down. The melting point of a pure substance is a characteristic physical property and can be used to identify the substance or assess its purity (impurities lower the melting point). Substances with stronger forces between particles (e.g. giant ionic or giant covalent structures) have very high melting points; substances with weak intermolecular forces (e.g. simple molecular compounds) have low melting points.
Exam point: A pure substance has a sharp, fixed melting point; an impure substance melts over a range of temperatures.
Sources
- Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025), Cambridge Assessment International Education
Common Misconceptions
| Misconception | Reality |
|---|---|
| The temperature keeps rising while a substance is melting. | The temperature of a pure substance stays constant at the melting point during melting; the energy supplied is used to overcome the forces of attraction between particles, not to raise the temperature. |
| The particles themselves melt, becoming softer or changing into different particles. | The particles are unchanged during melting; only their arrangement and movement change, as they gain energy and break free from their fixed positions in the lattice. Melting is a physical change. |