Melting
Melting is the process by which a solid changes into a liquid. The melting point is the temperature at which melting occurs; at a definite or constant temperature. A pure substance only melts at its melting point. The presence of impurities lowers the melting point of a substance. If a substance expands when it changes from a solid to a liquid, increased pressure will raise its melting point.
During the process of melting temperature change only occurs when the substance remains in the same state when heated, The heat absorbed by the solid, breaks down the intermolecular bonds between the molecules of the solid. When the bonds are broken the
molecules move from their fixed positions, and the solid has melted. During this change of state, no change of temperature has occurred.
Solidification
Solidification (or freezing) is the process by which a liquid turns into a solid. The freezing point of a substance is the temperature at which freezing occurs, which is the same as its melting point (in a pure substance).
During the change of state from liquid to solid, there is no change in temperature even though heat is lost to the surroundings as the substance cools. This heat is released as the intermolecular bonds are formed when the liquid atoms or molecules come together to form a solid. Temperature changes only when the substance is cooled in the same state.
Effects of impurities and pressure
Impurities in a substance lower its melting and freezing point. E.g. salt is put onto roads to prevent the freezing of ice. It also lowers the freezing point of water.
Putting pressure on ice lowers its melting point, e.g. when ice cubes or lumps of snow are pressed together, the parts of snow under high pressure begin to melt. When the pressure is released the water refreezes and therefore joins the two cubes or lumps together.