Melting and solidification

Melting is the change of state from a solid to a liquid. Melting of a pure substance occurs at a particular constant temperature called melting point.

The change of state from a liquid to a solid is called solidification or freezing or casting. A pure substance freezes at a temperature equal to its melting point.

In most of the substances, melting causes expansion and freezing causes contraction. Water is an exception. Ice melts to water causes contraction and water freezes to ice causes expansion. Density of water is highest at 4°C – water reaches minimum volume. This is why ice floats on water.

From the graph above,

• $0^{\circ}\text{C}$ is the melting point of the solid.
• Temperature remains constant at $0^{\circ}\text{C}$ as the solid is melting.
• During the melting process, solid and liquid exist in equilibrium.

Step by step process of what happens during melting:

1. Heat energy is absorbed by the particles
2. Heat energy is converted to kinetic energy
3. The kinetic energy of the particles increases and the particles in the solid vibrate faster
4. At melting point, the particles have gained enough energy to overcome the attractive forces between particles
5. Particles starts to move away from their fixed position
6.  Liquid is formed

Cause for constant temperature during melting: The absorbed heat energy during melting is used to weaken the attractive forces between particles and not the kinetic energy of the particles.

 

Factors That Affect Melting Point

Melting point is affected by purity of sample and pressure on the sample.

When impurities are mixed with a pure substance, the melting point is affected. This change in the melting point has its usefulness:

• In cold countries, water in pipe lines tend to freeze in winter season, where the ambient temperature drops to below $0^{\circ}\text{C}$. Freezing causes the water in the pipe to expand (recall that water expands when it freezes) and this might cause the pipe to burst. The common method to prevent this is to add antifreeze. With the addition of antifreeze, the melting point of the water + antifreeze mixture will drop to below $0^{\circ}\text{C}$ and hopefully, below the ambient temperature.
• Adding salt to water can reduce its melting point to as low as -18 °C. Salt is put onto the roads in cold countries during the winter season.

 

Mostly substance increases their melting point when a pressure is applied in their solid state.

• Normal solids such as iron, copper undergoes expansion when they melt. When pressure is applied on the surface of a normal solid, the expansion is suppressed and melting is delayed. Thus, the melting point of a normal solid is raised by the application of pressure.
• Abnormal solids, like ice and bismuth, contract on melting into liquids. When pressure is applied on the surface of such a solid, the change into the liquid is assisted by the increase in pressure. Thus, the melting point of ice is lowered by the application of pressure.

Even though the examples above are metals, the change in melting point due to application of pressure occurs for non-metals as well. An example will be ice. With the addition of pressure, the melting point of ice will be lowered.

 

Note: Freezing point of pure water is 0 °C at standard atmospheric pressure. (Melting point of a substance must be stated together with its purity and surrounding pressure.)