Chemistry · Solutions · NEET
An ideal solution follows Raoult's law over the whole range of composition. When you mix it there is no heat given out or taken in (ΔmixH = 0) and no change in total volume (ΔmixV = 0). A non-ideal solution does NOT follow Raoult's law. Its real vapour pressure is either higher (positive deviation) or lower (negative deviation) than what Raoult's law predicts, and mixing has ΔH ≠ 0 and ΔV ≠ 0. This is one of the most repeated ideas in NEET, so learn it as a clean checklist.
In an ideal solution of A and B, the attraction between A and B molecules is almost the SAME strength as A–A and B–B attractions. So when you mix them, no new stronger or weaker bonds form. Because the forces are unchanged, no heat is released or absorbed (ΔH = 0) and the molecules pack the same way, so the volume does not change (ΔV = 0). This equal-force idea is the single reason behind both zeros.
No. ΔmixG is NOT zero. For any solution that forms on its own, mixing is spontaneous, so ΔmixG is negative (less than 0). Also ΔmixS is positive (greater than 0) because mixing increases disorder. Only ΔmixH = 0 and ΔmixV = 0. NEET set this exact trap in 2016 and 2019 — many students wrongly pick ΔG = 0.
No, this is a common wrong idea. Molecules in an ideal solution DO attract each other. The point is that A–B attraction is nearly equal in strength to the average of A–A and B–B attractions, so there is no NET change on mixing. Saying 'no interactions occur' is false — ReNEET 2026 used this exact wrong reason to trick students.
Ideal (nearly): benzene + toluene, n-hexane + n-heptane, chloroethane + bromoethane. These molecules are similar in size and force, so A–B ≈ A–A ≈ B–B. Non-ideal: acetone + chloroform (negative deviation, they form a new H-bond), ethanol + acetone or ethanol + water (positive deviation, H-bonds are broken). Learn the standard pairs by heart because NEET asks them directly.
Deviation means the real vapour pressure of the mixture is different from the value Raoult's law predicts. If the real value is HIGHER than predicted, it is positive deviation (A–B forces weaker, ΔH positive). If the real value is LOWER than predicted, it is negative deviation (A–B forces stronger, ΔH negative). Ideal solutions have zero deviation. The next step is to master these deviations in detail.
Which one of the following is incorrect for an ideal solution?
5 moles of liquid X and 10 moles of liquid Y make a solution having a vapour pressure of 70 torr. The vapour pressures of pure X and Y are 63 torr and 78 torr respectively. Which of the following is true regarding the described solution?
Assertion A: For an ideal solution formed by mixing liquids P and Q, ΔmixH = 0 and ΔmixV = 0. Reason R: No interactions occur between P and Q. Choose the most appropriate answer:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It obeys Raoult's law over the full composition range, ΔmixH = 0 (no heat change on mixing), and ΔmixV = 0 (no volume change on mixing).
Benzene + toluene and n-hexane + n-heptane are classic ideal (or nearly ideal) solutions because the molecules are similar and A–B forces equal A–A and B–B forces.
No. ΔmixG is negative because mixing is spontaneous. Only ΔmixH and ΔmixV are zero. ΔmixS is positive. NEET tests this often.
Unequal A–B forces. If A–B is weaker than A–A/B–B you get positive deviation (higher vapour pressure). If A–B is stronger you get negative deviation (lower vapour pressure).
No. Only non-ideal solutions with large positive or negative deviation form azeotropes (minimum-boiling or maximum-boiling constant-boiling mixtures). Ideal solutions can be fully separated by fractional distillation.
