'Adsorption' principle is used for which of the following purification method?

Step 1: Define Adsorption in Purification Context Adsorption is the process by which molecules (adsorbate) stick to the surface of a solid (adsorbent). Different compounds adsorb with different strengths based on their polarity and the nature of the adsorbent.

Step 2: Match Each Method to Its Principle | Method | Principle | |---|---| | Extraction | Differential solubility (partition) in two immiscible solvents | | Chromatography ✅ | Differential adsorption on a solid stationary phase | | Distillation | Differences in boiling points (vapour pressure) | | Sublimation | Direct solid ↔ vapour conversion (vapour pressure) |

Step 3: How Adsorption Chromatography Works In column chromatography and TLC:

• Stationary phase: solid adsorbent ($\ce{SiO2}$ or $\ce{Al2O3}$)
• Mobile phase: liquid solvent
• Components with different polarity adsorb with different strengths
• More polar compound → stronger adsorption → slower movement → lower $R_f$
• Less polar compound → weaker adsorption → faster movement → higher $R_f$

Step 4: Conclusion

Answer: (b) Chromatography

Key Points to Remember:

• Adsorption uses a solid stationary phase; partition uses a liquid stationary phase
• Adsorption order on polar adsorbent: $\text{COOH} > \text{OH} > \text{C=O} > \text{COOR} > \text{C-X} > \text{aromatic} > \text{alkene} > \text{alkane}$
• Silica gel ($\ce{SiO2 . xH2O}$) and alumina ($\ce{Al2O3}$) are the standard adsorbents
• Activated charcoal is a non-polar adsorbent (special case)