Why non ideal solution show positive and negative deviations?

 Ideal Solution 

Those solution which obey Rault's law in its entire range of concentration and for such solution observed value of pressure will always equal to the calculated value of vapour pressure. 

Characteristics of Ideal Solution

• In the ideal solution ∆S mixing always be increasing that is ∆S equal to positive. 
• In the ideal solution ∆G value always be decreasing so ∆G is equal to negative.
• In the ideal solution ∆V mixing always be equal to zero.  
• In the ideal solution ∆H mixing always be zero. 
• In the ideal solution the force of attraction between A-B molecule will always be equal to the force of attraction between A-A molecule and B-B.  

Graphical Representation of Ideal Solution  

Non Ideal Solution  

Solution which not obey Rault's law in its entire range of concentration it's means for such solution observed value of pressure not equal to the calculated value of pressure. 

Non ideal solution which show positive deviation form Rault's Law  

If observed value of vapour pressure is greater than calculated value of vapour pressure according to Rault's law then such solution is known as non ideal solution which showing positive deviation from Rault's Law.  

Characteristics of Non Ideal Solution  

• The force of attraction between A-B molecule is lesser than A-A molecule or B-B molecule than such type of solution shows positive deviation with respect to Rault's Law and for such solution above vapour pressure value will be greater than calculated value.  
• ∆V mixing always be greater than zero. 
• ∆H mixing always be greater than zero. 

Non ideal solution which showing negative deviations from Rault's Law  

• If the force of attraction between A-B molecule is greater than force of attraction than such type of solution shows negative deviations with respect to Rault's and for such solution vapour pressure value will be lesser than calculated pressure value. 
• ∆V mixing always be lesser than zero.
• ∆H mixing always be lesser than zero. 

Azeotrophic Mixture 

• Those a mixture which cannot be separated even with the help of distillation process known as asotrophic mixture. 
• The reason is that the both component of binary solution not have sufficient difference in their boiling point rather they have very closer boiling point. 
• As you tropic mixture has same composition a liquid as well as before they are also known as constant volume point mixture. 

They are of two types 

1. Maximum Boiling Azeotrphs 

Those Azeotrphs mixture which show negative deviation from Rault's Law known as maximum boiling azeotrophic mixture. 

2. Minimum Boiling Azeotrphs

Those azeotrophic mixture with so positive deviation from Rault's Law known as minimum boiling azeotrophic mixture.