Question

Non - ideal solutions exhibit either positive or negative deviations from Raoult's law. What are these deviations and why are they caused?

Answer 1

Hint: Raoult's theorem states that the vapour pressure of a part in a solution at a given temperature is proportional to the mole fraction of that part in the solution multiplied by vapour pressure of that part in the pure state.

Complete answer: A non-ideal solution is one that deviates from the ideal solution's laws, in which the interactions between molecules are similar (or rather close) to those between molecules with different materials. There are mainly two types of deviation:
Positive divergence from Raoult's law:
Positive divergence from Raoult's law occurs when the partial vapour pressure of each factor (A and B), and hence the overall vapour pressure, is greater than the pressure predicted by Raoult's law.
Cause :
The solution in which the forces of attraction between A-A molecules and between B-B molecules are greater than the forces of attraction between A-B molecules is observed as a positive deviation.
${\gamma _{A - B}} < {\gamma _{A - A}}$ or ${\gamma _{B - B}}$
Negative deviation from Raoult's law:
Negative deviation from Raoult's law occurs where the partial vapour pressure of each portion of the solution is less than the vapour pressure predicted by Raoult's law.
Causes :
The Solutions in which the forces of attraction between A-A and B-B molecules are smaller than the forces of attraction between A and B molecules display this kind of deviation.
${\gamma _{A - B}} > {\gamma _{A - A}}$ or ${\gamma _{B - B}}$

Note:
Raoult's law could be used to determine the lowering of vapour pressure for non-volatile solutes, as well as to measure the strength of liquid bonding. Raoult's rule has a limitation in that it only applies to very dilute solutions. Only solutions containing non-volatile solutes are subject to Raoult's law.