Concept of Solution
There are several ways by which concentration of solution is quantitatively described: molality, molarity, mole fraction, ppm, mass % etc. Liquid- liquid solutions can be classified as ideal and non-ideal solution on the basis of Raoult's law. Suppose the molecules of two components are represented by A and B respectively. In pure components, the intermolecular attractive interaction will be of types A-A and B-B, whereas in the binary solution, A-B type of interactions will also be present. If intermolecular attractive forces between the A-A and B-B are nearly equal to those between A-B, ideal solution is formed.
The properties of solution which depend on the number of solute particles irrespective of their nature relative to the total number of particles present in the solution are called colligative properties.
The solution of acetone in ethanol:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → shows positive deviation from Raoult's law.
The passage compares the A–A and B–B attractions in the pure liquids with the A–B attractions in the solution. Nearly equal attractions give an ideal solution. When they are not equal, the solution deviates from Raoult’s law:
Positive deviation: A–B attractions are weaker than A–A and B–B. Molecules escape more easily, so the vapour pressure is higher than Raoult’s law predicts (ΔmixH > 0, ΔmixV > 0).
Negative deviation: A–B attractions are stronger. Molecules are held back, so the vapour pressure is lower than predicted (ΔmixH < 0, ΔmixV < 0).
In pure ethanol, the molecules are held together by hydrogen bonds (O–H…O). Acetone molecules get in between the ethanol molecules and break some of these hydrogen bonds. The attractions in the mixture are weaker than in pure ethanol, so both liquids escape more easily and the solution shows positive deviation.
Why the other options are incorrect
Option (2) shows negative deviation from Raoult's law. — Incorrect. Negative deviation needs stronger A–B attractions. An example is chloroform + acetone, where the H of chloroform forms a new hydrogen bond with the C=O oxygen of acetone.
Option (3) obeys Raoult's law. — Incorrect. A solution obeys Raoult’s law only when the attractions stay nearly the same, as in n-hexane + n-heptane or benzene + toluene. Here hydrogen bonds are broken, so the attractions change.
Option (4) shows both negative and positive deviation from Raoult's law. — Incorrect. The direction of deviation is fixed by whether the A–B attractions are weaker or stronger than in the pure liquids. For ethanol + acetone they are weaker, so the deviation is positive only.
NCERT (2026-27 edition):
Non-ideal Solutions – “The cause for these deviations lie in the nature of interactions at the molecular level. In case of positive deviation from Raoult’s law, A-B interactions are weaker than those between A-A or B-B, i.e., in this case the intermolecular attractive forces between the solute-solvent molecules are weaker than those between the solute-solute and solvent-solvent molecules. This means that in such solutions, molecules of A (or B) will find it easier to escape than in pure state. This will increase the vapour pressure and result in positive deviation. Mixtures of ethanol and acetone behave in this manner. In pure ethanol, molecules are hydrogen bonded. On adding acetone, its molecules get in between the host molecules and break some of the hydrogen bonds between them. Due to weakening of interactions, the solution shows positive deviation from Raoult’s law [Fig. 1.6 (a)].”
