Physical Chemistry

Characteristics of physisorption:

• Non-specific: It arises from weak van der Waals forces, which exist between any gas and solid (correct characteristic).
• Low enthalpy: Because of weak forces, the enthalpy of physisorption is quite low, ranging only between $20 - 40\text{ kJ mol}^{-1}$. In contrast, chemisorption has high enthalpy of adsorption ($80 - 240\text{ kJ mol}^{-1}$). Therefore, stating that 'Enthalpy of adsorption is high' is NOT a characteristic of physisorption.
• Multimolecular layers: Under high pressure, gas molecules can form multiple layers on the solid surface (correct characteristic).

Colloidal sols are classified by surface charge:

• Metal sols (such as gold, silver, platinum), metal sulphides ($\text{As}_2\text{S}_3$, $\text{Sb}_2\text{S}_3$), and lyophilic sols like starch and gum are negatively charged or neutral.
• Hydrated metallic oxides ($\text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O}$, $\text{Al}_2\text{O}_3\cdot x\text{H}_2\text{O}$) and basic dye stuffs (methylene blue) are positively charged.
• In blood, hemoglobin carries a positive charge while serum proteins (albumin) carry a negative charge, leading to ambiguity. Because none of the given simple options unequivocally represented a standard positively charged sol without conflicting biological context, NTA officially DROPPED this question and awarded full credit to all candidates.