CBSE 2025, Set 3

Question

Shweta mixed two liquids A and B of 10 mL each. After mixing, the volume of the solution was found to be 20.2 mL.

(i) Why was there a volume change after mixing the liquids?

(ii) Will there be an increase or decrease of temperature after mixing?

(iii) Give one example for this type of solution.

Edvaya · Accepted Answer

The correct answer is Subjective. The final volume (20.2 mL) is **greater than** the expected 20.0 mL → **increase in volume on mixing**. **(i) Why volume increases:** This indicates **positive deviation from Raoult's law** — the A–B intermolecular interactions are **weaker** than A–A and B–B interactions in pure liquids. When A and B molecules mix, the molecules are held less tightly together, resulting in a slight **increase in volume** ($\Delta V_{mix} > 0$). **(ii) Temperature change:** Since A–B interactions are weaker than in pure components, mixing is **endothermic** ($\Delta H_{mix} > 0$) — energy is absorbed to break A–A and B–B interactions but less energy is released forming A–B bonds. Therefore, the **temperature decreases** after mixing. **(iii) Example of such a solution:** **Ethanol and acetone** (or any other pair showing positive deviation: benzene + toluene also works, but ethanol + acetone is the standard CBSE answer).

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