What is Positive Deviation from Raoult’s Law?

Raoult’s Law states that in an ideal solution, the partial vapor pressure of each component is directly proportional to its mole fraction. However, in real solutions, deviations occur. When the observed vapor pressure is higher than expected, it is called a positive deviation from Raoult’s Law.

This happens when the intermolecular forces between the components (A–B) are weaker than those in the pure liquids (A–A and B–B). As a result, molecules escape more easily into the vapor phase, increasing vapor pressure.

Examples include ethanol and acetone, or acetone and carbon disulfide mixtures. In these solutions, the hydrogen bonding between ethanol molecules is broken when mixed with acetone, leading to weaker interactions and hence more vaporization.

In positive deviation:

• The solution shows higher vapor pressure than predicted.
• The boiling point decreases.
• The enthalpy of mixing (ΔHmix) is positive because energy is absorbed to break existing bonds.

Thus, positive deviation indicates that the solution behaves less ideally, with weaker intermolecular attraction between the components than in their pure states.