Illustrate and demonstrate that, for correct mass balance, relative amount of two co-existing phases or micro constituents should be as given by lever rule.

The lever rule gives fractions of two co-existing phases. The composition of every phase can be found by the lever rule.

Vertical line AB is drawn on the composition scale as shown in fig. Intersection of this line (point M) with the temperature line (tie line) is the fulcrum of simple lever arm. Length LM and MN indicate the amount of solid and liquid respectively.

Where LM + MN = LN = Total composition of an alloy at temperature t.