The adsorbent used in adsorption chromatography is/are A. silica gel B. alumina C. quick lime D. magnesia Choose the…

Step 1: Requirements for a Good Adsorbent

An adsorbent for chromatography must be:

1. Chemically inert (must not react with compounds being separated)
2. Porous with large surface area (for maximum adsorption)
3. Available in appropriate polarity

Step 2: Evaluate Each Option

A. Silica gel ($\ce{SiO2 . xH2O}$) ✅

• Polar adsorbent with surface silanol ($\ce{Si-OH}$) groups
• Surface area: 200–600 m²/g
• Standard adsorbent in column chromatography and TLC
• Adsorption order: carboxylic acids > alcohols > ketones > esters > hydrocarbons

B. Alumina ($\ce{Al2O3}$) ✅

• Polar adsorbent; available in acidic (pH 4–4.5), neutral (pH ~7), and basic (pH 9–10) forms
• Surface area: 150–400 m²/g
• Used for separating alkaloids, steroids, and pigments

C. Quick lime ($\ce{CaO}$) ❌

• Too reactive — reacts with water (forming $\ce{Ca(OH)2}$) and acidic organic compounds
• Used as a drying agent and in cement manufacturing
• NOT used in chromatography

D. Magnesia ($\ce{MgO}$) ❌

• Not a standard chromatography adsorbent
• Limited use in carotenoid separation only
• Very rarely encountered; not relevant for JEE purposes

Step 3: Conclusion

Only silica gel and alumina are standard adsorbents → Answer: (c) A and B only

Key Points to Remember:

• Silica gel: most widely used; available in various mesh sizes (60–230 mesh)
• Alumina: three forms — acidic, neutral, basic (choose based on compound stability)
• Quick lime and magnesia are NOT adsorbents for chromatography
• Activated charcoal = non-polar adsorbent (used for decolorisation and separating non-polar compounds)