What is tie-line rule?
The tie-line rule is applied to verify the compositions of two co-existing phases in a binary phase diagram. The tie line is a horizontal line drawn at the temperature of interest within the two phase region. The tie line rule is not concerned with phases. It can be applied only in the two stage region. The overall compositions of two co-existing phases remain similar on the tie-line. There is change only in their relative amounts which is explained by the lever rule.