Melting and Freezing: Liquid-Solid Transitions

Melting is the change of a solid into a liquid, and freezing is the change of a liquid into a solid. Both happen at the same temperature for a pure substance, called the melting point (or freezing point). In brief: pure water ice melts at 0 °C at normal pressure, and during melting the temperature stays at 0 °C until all the ice has become liquid.

Melting and freezing are physical changes: the substance keeps its identity and only its state changes. This article explains what happens to particles during these transitions, why the temperature stays constant, how much heat is needed, how to read a heating curve and how melting is used in daily life.

What happens during melting?

When a solid is heated, its particles vibrate more strongly. At the melting point, the particles have enough energy to break out of their fixed positions and begin to slide past each other. The orderly solid structure collapses, and the substance becomes a liquid.

Energy is absorbed during melting, so melting is an endothermic change. The energy is used to loosen the bonds between particles rather than to make them move faster.

What happens during freezing?

Freezing is the reverse process. When a liquid is cooled, its particles lose energy and move more slowly. At the freezing point they settle into fixed positions, and the liquid becomes a solid. Energy is released to the surroundings, so freezing is exothermic.

For a pure substance, the freezing point and the melting point are the same temperature. Water freezes and ice melts at 0 °C, so which process occurs depends on whether heat enters or leaves.

Why does the temperature stay constant?

While a pure solid is melting, you keep adding heat, yet a thermometer shows a constant reading. All the added energy goes into overcoming the attractions between particles. Only when the last of the solid has melted does the temperature of the liquid begin to rise again.

The same is seen during freezing: the temperature of the mixture stays at the freezing point until all the liquid has become solid. This constant temperature is a good test of purity. Impure substances usually melt over a range of temperatures and at a lower temperature than the pure substance.

Melting points of common substances

How do you read a heating curve?

A heating curve plots the temperature of a substance against the time of heating at a steady rate. For water starting as ice at −10 °C, the curve has five parts.

The flat sections show changes of state, and the sloping sections show heating within one state.

Worked example: heat needed to melt ice

The energy needed to melt a solid at its melting point is Q = m × L_f, where m is the mass and L_f is the specific latent heat of fusion. For water, L_f is about 334 J/g (334 kJ/kg).

1. Melting 50 g of ice at 0 °C. Q = 50 g × 334 J/g = 16,700 J, or 16.7 kJ.
2. Warming this water from 0 °C to 20 °C. Using the specific heat capacity of water, 4.18 J/(g·°C): Q = 50 × 4.18 × 20 = 4,180 J.
3. Total energy. 16,700 J + 4,180 J = 20,880 J, or about 20.9 kJ.

Notice that melting the ice takes about four times more energy than warming the same mass of water by 20 °C. This is why an ice cube keeps a drink cold for so long.

Where are melting and freezing used?

• Metalworking: metals are melted and poured into molds, where they freeze into shape.
• Food: chocolate melts near body temperature, which is why it melts in your mouth, and ice cream is made by freezing a mixture.
• Road safety: salt spread on icy roads lowers the freezing point of water, so ice melts at temperatures below 0 °C.
• Soldering: a low-melting alloy joins electronic parts.

How were melting points established?

Early thermometers needed fixed reference points. In the 1700s the freezing and boiling points of water were chosen as reference marks for temperature scales: Anders Celsius originally proposed a scale based on these two points. Joseph Black studied heat during melting in the 1760s and introduced the idea of latent heat, showing that heat can be absorbed without a rise in temperature.

Which mistakes are common?

Mistake 1: Thinking that the temperature rises while ice melts

It stays at 0 °C until all the ice has melted, provided the mixture is well stirred.

Mistake 2: Confusing melting with dissolving

Melting needs heat and gives a liquid of the same substance. Dissolving needs a solvent.

Mistake 3: Saying that freezing absorbs heat

Freezing releases heat. Orchard growers sometimes spray water on blossoms before a frost, because the freezing water releases heat that protects the buds.

Mistake 4: Believing that all solids have a sharp melting point

Pure crystalline solids do. Mixtures, glass and many plastics soften gradually over a range of temperatures.