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.
Solution
Positive deviation: the observed vapour pressure is higher than Raoult's law predicts. Cause: A–B attractions weaker than A–A and B–B, so molecules escape more easily. Example: ethanol + acetone (acetone breaks ethanol's hydrogen bonds).
Negative deviation: the observed vapour pressure is lower than predicted. Cause: A–B attractions stronger than A–A and B–B (often a new hydrogen bond). Example: chloroform + acetone, where a H-bond forms between CHCl's H and acetone's O.
Negative deviation: the observed vapour pressure is lower than predicted. Cause: A–B attractions stronger than A–A and B–B (often a new hydrogen bond). Example: chloroform + acetone, where a H-bond forms between CHCl's H and acetone's O.
CBSE Class 12 Board · 2010 · Delhi · Subjective