Solutions With Negative Deviation

In this type of deviation, the partial vapor pressure of each component of solution is less than the vapor pressure as expected according to Raoult's law.

fig 3.6

Solvent-solute interaction is greater than solute-solute and solvent- solvent interaction. Therefore the escaping tendency decreases which results in the decrease in their partial vapor pressures. In such solutions, total vapor pressure of the solution is also less than the vapor pressure expected according to Raoult's law. An e.g., of solution exhibiting negative deviation is chloroform and acetone, the molecular interaction of which can be shown as below.

Since a new force of attraction exists between the molecules, the partial vapor pressure decreases.

Examples of negative (-ve) deviation,

Acetic acid and pyridine

1) CH₃COOH + C₅H₅NChloroform and acetone

2) CHCl₃ + (CH₃)₂COCloroform and benzene

3) CHCl₃ + C₆H₆Chloroform and Diethylether

4) CHCl₃ + (C₂H₅)₂OWater and hydrochloric acid gas

5) H₂O + HClAcetone and aniline

6) (CH₃)₂CO + C₆H₅ NH₂