# What is Lever Rule ?

The rule which helps to calculate the relative proportions of liquid phase and solid phase present in the binary mixture at any given temperature is known as Lever rule.

Let, composition of liquid phase is C_{L}, composition of solid phase is C_{S} and the composition of (S + L) is C_{o} in the binary mixture.
Then -

**Fraction of Solid Phase = (C _{L} − C_{o})/(C_{L} − C_{S})**

Fraction of Liquid Phase = (Co − C_{L})/(C_{L} − C_{S})

From these formula we can calculate the composition of liquid and solid phase in two phase system using phase diagram.

Let us assume we have a binary mixture (alloy) system where the phase diagram shows:

Composition of C_{S} = 10% alloy

Composition of C_{L} = 40% alloy

The overall (S + L) composition C_{o} = 25% alloy

Applying the Lever Rule:

Fraction of C_{S} = (C_{L} − C_{o})/(C_{L} − C_{S})

or, Fraction of C_{S} = (40 − 25)/(40 − 10) = 0.5

So, C_{S} constitutes 50% of the total.

Similarly, Fraction of C_{L} = (C_{o} − C_{L})/(C_{L} − C_{S})

or, Fraction of C_{S} = (25 − 10)/(40 − 10) = 0.5

So, C_{L} constitutes 50% of the total.