The purification method based on the following physical transformation is :

Step 1: Identify the Physical Transformation

The diagram shows the sequence: Solid → (on heating) → Vapour → (on cooling) → Solid

This is a direct solid ↔ vapour transition with no liquid phase involved at any stage.

Step 2: Identify the Purification Technique

This is Sublimation — the process in which a solid directly converts to vapour on heating, and the vapour directly converts back to solid on cooling, completely bypassing the liquid state.

Step 3: Compare with Other Techniques

| Technique | Phase Transitions | |---|---| | Sublimation ✅ | Solid ↔ Vapour (no liquid intermediate) | | Crystallisation ❌ | Solid dissolves in liquid → recrystallises on cooling | | Extraction ❌ | Transfer of compound between two immiscible liquid solvents | | Distillation ❌ | Liquid → Vapour → Liquid (liquid phase essential) |

Step 4: How Sublimation Works in Practice

1. The impure solid is heated in a sublimation apparatus
2. The substance with appreciable vapour pressure converts to vapour
3. Vapour passes to the cold finger (cooler surface) above
4. Pure sublimate collects on the cold surface
5. Non-sublimable impurities remain as residue

Answer: (a) Sublimation

Key Points to Remember:

• Sublimation requires the compound to have significant vapour pressure below its melting point
• Impurities must NOT sublime — this is the basis of purification
• Substances that sublime: camphor, naphthalene, iodine, benzoic acid, ammonium chloride, anthracene
• A cold finger is used to collect the pure sublimate