Q.Mixture of ethyl alcohol and acetone shows positive deviation from Raoult's law. Explain why. OR The boiling point of a pure solvent increases when a non-volatile, non-electrolyte solute is added to it. Explain with the help of vapour pressure versus temperature graph.

Ideal and Non-Ideal Solutions

Imagine you have two friends, A and B. When they work together, sometimes they get along perfectly — each does exactly their share, no extra effort, no friction. Other times, they either pull apart (making the job harder) or cling together (making it easier than expected). A solution of two liquids behaves the same way.

The Intuition: What "Ideal" Really Means

An ideal solution is the "perfect teamwork" case. The molecules of A and B are so similar that they don't care whether they are next to an A or a B. They interact with each other exactly as they would with their own kind. Think of mixing two grades of petrol — the molecules are nearly identical, so the mixture behaves predictably.

In an ideal solution, two things happen:

• No heat is absorbed or released when you mix them (zero enthalpy change, $\Delta H_{\text{mix}} = 0$).
• No volume change occurs — the total volume is exactly the sum of the individual volumes ($\Delta V_{\text{mix}} = 0$).

Why? Because the forces between A-A, B-B, and A-B are all the same. No energy is needed to break old contacts or form new ones; no space is saved or wasted.

The Precise Statement: Raoult's Law

Raoult's law is the mathematical definition of an ideal solution. For a mixture of two volatile liquids A and B, the partial vapour pressure of each component above the solution is proportional to its mole fraction in the liquid:

where $p_A^0$ and $p_B^0$ are the vapour pressures of pure A and pure B at that temperature. The total vapour pressure is simply the sum:

This is a straight line when plotted against mole fraction. Every ideal solution obeys this law at all compositions and temperatures.

Non-Ideal Solutions: When Molecules Disagree

Real solutions are rarely ideal. The molecules of A and B are different — different sizes, polarities, or hydrogen-bonding abilities. Now the A-B interaction is not the same as A-A or B-B. This causes deviations from Raoult's law.

Positive deviation occurs when A-B interactions are weaker than A-A and B-B. The molecules "prefer their own company" and escape into the vapour more easily than expected. The actual vapour pressure is higher than Raoult's law predicts. Examples: ethanol + water (initially), acetone + carbon disulphide. The mixture absorbs heat ($\Delta H_{\text{mix}} > 0$) and expands slightly ($\Delta V_{\text{mix}} > 0$).

Negative deviation occurs when A-B interactions are stronger than A-A and B-B. The molecules "hold on to each other" and escape less easily. The actual vapour pressure is lower than Raoult's law predicts. Examples: chloroform + acetone, nitric acid + water. The mixture releases heat ($\Delta H_{\text{mix}} < 0$) and contracts ($\Delta V_{\text{mix}} < 0$).

The Key Table