Melt is a physical process whereby a material changes from a solid to a liquid state. The temperature at which this occurs is known as the melting point. At the melting point, the molecules of the material are in constant motion and are able to slide past one another, allowing the material to take on a liquid form.
Materials can exist in three different states: solid, liquid, or gas. The state of matter is determined by the strength of the forces that hold the particles of the substance together. In a solid, these forces are very strong and keep the particles close together in a fixed position. In a liquid, these forces are weaker than in a solid, but still strong enough to keep the particles close together. In a gas, these intermolecular forces are so weak that they cannot keep the particles from moving about independently.
The amount of energy required to change the state of matter depends on how much mass there is and how far apart the particles are. For example, it takes less energy to vaporize (or boil) water than it does to melt ice because there is less mass (fewer water molecules) and because water molecules are further apart in ice than they are in liquid water. Similarly, it requires more energy to melt lead than iron because lead has more mass per molecule and its atoms are closer together than those of iron.
The melting point of a substance can vary depending on what other substances it is mixed with (its impurities). For example, adding salt to ice lowers its freezing point—the temperature at which it changes from liquid back to solid—because salt dissolves into water molecules and disrupts their ability to bond with one another as strongly as they do when pure water freezes.