# Raoults law 7. Raoult's law is not valid in.  (1) Non- ideal solution showing positive deviation. (2) Non-ideal solution showing negative deviation.  (3) Mixture of two immiscible liquid.  (4) all of these

Dear Student,

Ideal solutions are such where the two liquids do not interact with each other and the vapour pressure against composition graph is a straight line. Positive deviation from the Raoult's law depicts that the vapour pressure of the final solution will always be higher than expected from an ideal mixture.
This happens because the intermolecular forces of A-A and B-B in pure solutions is higher than the intermolecular forces of A-B in the mixture. As a result the molecules are held loosely in the mixture and have a higher vapour pressure than before. Also, less heat is evolved when the new attractions between A and B are set up than was absorbed to break the original ones (A-A and B-B​ bonds). Heat will therefore be absorbed when the liquids mix so the reaction is endothermic and enthalpy of mixing is positive in this case.

For example a mixture of ethanol and water show positive deviation from Raoult's law.
Mixture of CCl4 and CHCl3 , acetone and ethyl alcohol,ethanol and water shows positive deviations.

Mixture of carbon disulphide to the acetone shows positive deviation from Raoult’s Law.Because

When carbon disulphide is added to the acetone, this weaken the dipolar interaction between molecule of solute solvent (A – B) than the respective interaction between solute-solute (A – B) and solvent – solvent (B – B), which leads to increase in vapour pressure. And mixture shows positive deviation in vapour pressure than Raoult’s Law. Negative deviation from the Raoult's law depicts that the vapour pressure of the final solution will always be lower than expected from an ideal mixture.
This happens because the intermolecular forces of A-A and B-B in pure solutions are lower than the intermolecular forces of A-B in the mixture. As a result the molecules are held more tightly in the mixture and have a lower vapour pressure than before. Also, more heat is evolved when the new attractions between A and B are set up than was absorbed to break the original ones (A-A and B-B​ bonds). Heat will therefore be released when the liquids mix so the reaction is exothermic and enthalpy of mixing is negative in this case. For example a mixture of nitric acid and water shows negative deviation from Raoult's law as stronger ionic attractions are set up between the ions.
H2O + HNO3 ⇌ H3O+ + NO3- So, ​ when the solution deviates positively from Raoult's law. when the solution deviates negatively from Raoult's law.

Now,  Raoults law is applicable only for dilute solutions not applicable to  immiscible liquid because immiscible liquids form layers, while this law is applied when the interactions between solute and solvent molecules is almost equal to those between each solute-solute and solvent-solvent molecule.When solution is dilute the particles per unit volume decreases so much, that we can almost neglect the difference in interaction energies between those two different molecules (in reality) and that brings "idealness" required for raoult's law to be applicable.

Hence, option Option 3 is correct.

Regards,

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