Activity of solution

In dealing with chemical reactions in solution it is important to define the activity of a component. Activity of a component denotes its effective concentration. It is related to fugacity as

(5)

  is the fugacity of component i in solution and  is the fugacity of a component in its standard state (standard state could be either pure element or compound at 1 atmospheric pressure) So at standard state activity equals 1. In an ideal gas activity of a component i is equal to its partial pressure.

Raoults’s Law

An ideal solution obeys Raoult’s law, in which activity of a component ai equals to its mole fraction       

(6)

                                                                                                                          
Real solutions exhibit either positive or negative deviation from Raoult’s law for a binary solution. Deviation from Raoult’ law is taken care by activity coefficient

(7)

The Fe-Mn forms an ideal solution, whereas the Fe-Cu exhibits strong positive deviation and the Fe-Si strong negative from Raoult’s law. Physically it implies that in Fe-Cu solution copper has a strong tendency to segregate, and in Fe-Si solution silicon has a strong tendency to form chemical compound with iron.

In binary liquid oxides, FeO-MnO behaves ideally, whereas most binary silicates i.e. show negative deviation from Raoult’s law.