Equilibrium Diagrams

When a pure metal solidifies it changes from a liquid to a solid state. An intermediate state of liquid and solid exists (sometimes known as the pasty state). These states are known as phases, a phase being defined as regions that differ from one another, either in composition or in structure or in both. In a metal the liquid state consists of atoms randomly arranged whereas in the solid state the atoms are arranged regularly in crystal lattices. Therefore the structure of the two states is different and is referred to as phases.
When a pure metal is cooled from the liquid state is produces a cooling curve as in Figure 1. The change from the liquid to the solid state occurs at a definite temperature. Where solidification begins and finishes at the same temperature but the time increases. Examples of metals that have this are lead, copper, aluminum. A pure metal solidifies at one fixed temperature, a fact which can be checked by plotting a cooling curve. A cooling curve may be obtained by melting a small amount of a metal and recording the temperature drop at suitable time intervals as this metal solidifies (the metal must be allowed to cool very slowly i.e. under equilibrium conditions) . We can then plot a graph of temperature against time to give us the cooling curve for that particular metal.
At temperatures above and below the curve falls smoothly without "kinks". When the solidification temperature is reached, the temperature remains CONSTANT for some time thus giving rise to the step in the curve. Down to the temperature of the liquid drops in a regular manner as heat is being lost to the surroundings at a nearly constant rate. The step is due to latent heat. This leads to zero change in temperature until the last drop of liquid has solidified. After no more latent heat is available the solid continues to cool in a regular manner giving the smooth curve.