Melting and freezing refer to the changes in state which occur when the solid and liquid states interchange. Melting occurs when a solid is heated and turns to a liquid and freezing occurs when a liquid is cooled and turns to a solid.

Melting - solid to liquid
When a solid is heated, its particles gain enough energy to overcome the bonding forces holding them firmly in place. They start to move about, but stay close to their neighbouring particles, rolling around each other. This particle movement allows the substance to flow and form a liquid. Solids and liquids are similar in their density, that is, number of particles per unit volume. They differ only in the ability of the liquid to flow and change shape to occupy the shape of the bottom part of its container.

Melting point of pure and impure substances
For pure substances, the temperature at which melting and freezing occurs is quite sharp and is called the melting point of the substance. For impure substances, melting and freezing occur more gradually over a range of temperature. This is one way that chemists identify the purity of a substance; a pure substance will melt at a set temperature while the more impure a substance is, the more its melting point will vary over a range of temperatures.

Freezing - liquid to solid
Freezing occurs when a liquid is cooled and turns to a solid. Upon cooling, the particles in a liquid lose energy, stop moving about and settle into a stable arrangement, forming a solid. Freezing occurs at the same temperature as melting, hence, the melting point and freezing point of a substance are the same temperature. The melting/freezing point of a substance is defined as the temperature above which, the substance is liquid and below which, it is solid.

Melting points
Different solids have different melting points depending on the strength of bonding between the particles and the mass of the particles. Essentially, the heavier the particles in the solid, and the stronger the bonding, the higher the melting point.