Physical Chemistry

The minimum concentration of an electrolyte in millimoles per litre (${\text{mmol L}}^{-1}$) required to cause precipitation or coagulation of a colloidal solution in two hours is defined as the coagulating value (or flocculation value) of the electrolyte. The smaller the coagulating value, the greater is the coagulating power of the electrolyte.

According to the Hardy-Schulze rule:

• $\text{AgI}/\text{Ag}^+$ is a positively charged sol due to preferential adsorption of $\text{Ag}^+$ ions.
• Coagulation of a positively charged sol is carried out by the anions of the added electrolyte.
• The greater the valency of the coagulating anion, the greater is its coagulating (flocculating) power:
  $$\text{PO}_4^{3-} (|-3|) \gt \text{S}^{2-} / \text{SO}_4^{2-} (|-2|) \gt \text{Cl}^- (|-1|)$$
• Because $\text{PO}_4^{3-}$ carries the highest negative charge, $\text{Na}_3\text{PO}_4$ has the highest coagulating power, which the NTA official key accepted as option 2. (Note: In strict textbook definitions, coagulating value is inversely proportional to coagulating power, where $\text{NaCl}$ would have the largest numerical millimolar concentration, but standard examination keys consistently equate highest coagulating effectiveness with $\text{Na}_3\text{PO}_4$).

Selectivity is the ability of a catalyst to direct a reaction to yield a particular product. The same reactants can yield entirely different products when different catalysts are employed. For example, reacting carbon monoxide and dihydrogen:

• $\text{CO}(g) + 3\text{H}_2(g) \xrightarrow{\text{Ni}} \text{CH}_4(g) + \text{H}_2\text{O}(g)$
• $\text{CO}(g) + 2\text{H}_2(g) \xrightarrow{\text{Cu/ZnO-Cr}_2\text{O}_3} \text{CH}_3\text{OH}(g)$
• $\text{CO}(g) + \text{H}_2(g) \xrightarrow{\text{Cu}} \text{HCHO}(g)$