Freezing
Summary: The exothermic change of state from liquid to solid that occurs at a fixed temperature called the freezing point, equal to the melting point for a pure substance. Tags: igcse chemistry Created: 2026-07-17
Freezing (also called solidification) is the change of state in which a liquid becomes a solid. It is an exothermic process: energy is released to the surroundings as the substance freezes. As a liquid cools, its particles lose kinetic energy and slow down, and at the freezing point they no longer have enough energy to slide past one another. The forces of attraction between the particles then lock them into fixed positions, forming the regular lattice arrangement of a solid in which the particles can only vibrate. While a pure substance is freezing, its temperature stays constant at the freezing point, because the energy being released as the particles settle into fixed positions balances the energy being lost to the surroundings. For a pure substance, the freezing point is exactly the same temperature as the melting point — freezing and melting are the same change of state in opposite directions.
Freezing as a Change of State
Freezing is one of the interconversions between states of matter described by the kinetic particle theory. It is the reverse of melting.
Freezing: the change of state from liquid to solid, occurring at the freezing point, with energy released to the surroundings (exothermic).
What happens to the particles
| Stage | Particle behaviour |
|---|---|
| Liquid above the freezing point | Particles are close together, irregularly arranged, and move by sliding past one another. |
| Cooling the liquid | Particles lose energy to the surroundings; their kinetic energy decreases and they slow down. |
| At the freezing point | Particles no longer have enough energy to move past one another; the forces of attraction between them lock the particles into fixed positions. |
| Solid formed | Particles are closely packed in a regular lattice and can only vibrate about their fixed positions. |
Key points about the process:
- Freezing is exothermic: energy is transferred from the substance to the surroundings as the particles settle into the more ordered solid arrangement.
- During freezing, the particles lose kinetic energy and slow down until the attractive forces are strong enough, relative to the particles’ motion, to hold them in place.
- The temperature remains constant at the freezing point while the substance freezes. The energy released as particles lock into fixed positions replaces the energy being lost to the surroundings, so the average kinetic energy of the particles — and therefore the temperature — does not change until freezing is complete. This produces a flat, horizontal section on a cooling curve.
- Freezing is a physical change: the particles themselves are unchanged, no new substance is formed, and the change is reversed by melting.
Freezing point
The freezing point is the temperature at which a liquid changes to a solid. For a pure substance it is a sharp, fixed temperature at a given pressure.
For a pure substance, the freezing point is equal to the melting point; the two terms describe the same temperature approached from opposite directions:
- Freezing (liquid → solid): energy is released at this temperature.
- Melting (solid → liquid): energy is absorbed at this same temperature.
For example, pure water freezes at 0 °C and pure ice melts at 0 °C.
| Substance | Freezing point / melting point (°C) |
|---|---|
| Ethanol | −117 |
| Water | 0 |
| Sodium chloride | 801 |
| Iron | 1538 |
Substances with strong forces of attraction between their particles, such as ionic compounds and metals, freeze (and melt) at high temperatures; substances with weak forces between their molecules freeze at low temperatures.
Effect of impurities
Dissolved impurities lower the freezing point of a liquid and cause it to freeze over a range of temperatures rather than sharply at one temperature. This is why salt water freezes below 0 °C, and why salt is spread on roads in winter: the salt lowers the freezing point of water so that ice does not form (and existing ice melts) at temperatures a few degrees below 0 °C.
Freezing on a cooling curve
If a liquid is allowed to cool at a steady rate and its temperature is plotted against time, the graph shows:
- A falling line as the liquid cools (particles lose kinetic energy, temperature decreases).
- A horizontal (flat) section at the freezing point, while liquid and solid exist together and the energy released by particles locking into the lattice balances the energy lost to the surroundings.
- A falling line again once all the liquid has solidified and the solid continues to cool.
The temperature of the flat section is the freezing point of the substance, and for a pure substance it matches the melting point measured on a heating curve.
Sources
- Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025), Cambridge Assessment International Education
Common Misconceptions
| Misconception | Reality |
|---|---|
| Freezing means “getting cold”, so it absorbs energy. | Freezing is exothermic: energy is released to the surroundings as the particles slow down and lock into fixed positions. The substance loses energy, but the process gives energy out. |
| The freezing point and the melting point of a pure substance are different temperatures. | For a pure substance they are the same temperature; the name simply depends on the direction of the change. Pure water freezes at 0 °C and pure ice melts at 0 °C. |