Q4. Give the principle of (a) crystallisation, (b) distillation and (c) chromatography, with an example of each (NCERT Exercise 8.18).
(a) Difference in solubility of the compound and the impurities in a solvent, hot and cold: impure benzoic acid crystallised from hot water. (b) Difference in boiling points: chloroform (334 K) separated from aniline (457 K). (c) Difference in adsorption on, or partition between, a stationary and a mobile phase: plant pigments or amino acids separated by TLC or paper chromatography.
Q5. In a fractionating column, which component reaches the top first, and why does the vapour become purer as it rises?
The lower-boiling (more volatile) component. On each surface of the column the rising vapour meets descending liquid: the higher-boiling component condenses and the lower-boiling one revaporises, so every step, a theoretical plate, enriches the vapour in the more volatile component.
Q6. Two colourless compounds are separated by TLC. Give two ways to see where the spots are.
Look at the plate under ultraviolet light (fluorescing spots or UV-absorbing spots on a fluorescent plate show up), or put it in a closed jar with a few iodine crystals (compounds that adsorb iodine show as brown spots). A suitable spray reagent, such as ninhydrin for amino acids, also works.
Q7. Why must the two solvents in differential extraction be immiscible?
So that they form two separate layers in the separatory funnel, which can be run off one after the other. Miscible solvents would form one solution, and the compound could not be separated from the water.
Q8. A compound gives Rf = 0.55 with a solvent front 7.0 cm from the base line. How far from the base line is its spot?
Distance = Rf × solvent distance = 0.55 × 7.0 = 3.85 ≈ 3.9 cm.
Q9. Explain why an organic liquid vaporises below its boiling point in steam distillation (NCERT Exercise 8.28, page 294).
The liquid and water do not mix, so each exerts its own vapour pressure. The mixture boils when p1 + p2 equals the atmospheric pressure. Since p1 alone is less than that, boiling happens below the liquid’s own boiling point and below about 373 K at normal atmospheric pressure.