Non - ideal solutions exhibit either positive or negative deviations from Raoult's law. What are these deviations and why are they caused? Explain with one example for each type.

Answer

Positive deviations: In this type of deviation, the partial vapour pressure of each component (say `1` and `2`) of a solution is greater than the vapour pressure as expected accoring to Raoult's law.
The plot of vapour pressure of two component solutions as a function of mole fraction is shown below.
The intermolecular forces of attraction between solute-solvent molecules are weaker than those between solute-solute molecules and solvent -solvent molecules. Therefore, solvent molecules can easily escape, resulting in increse in vapour pressure.
Example Ethanol and acetone mixture
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Negative deviations: In this type of deviations, the partial vapour pressure of each component (say `1` and `2` ) of a solution is less than the vapour pressure as expected accoring to Raoult's law.
The plot of vapour pressure of two component solutions as a function of mole fraction is shown below.
The intermolecular forces of attraction between solute-solute molecules and solvent-solvent molecules are weaker than those between solute-solvent molecules. This results in the decreasing of vapour pressure.
Example: Chloroform and acetone mixture.
The intermolecular attractive forces between solute-solvent molecules increase due to the formation of `H-`
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