Excess insoluble reactant enables impurity removal by filtration

Steps:

• Identify process: Reaction produces soluble salt in solution with insoluble residue; filter residue, evaporate filtrate for pure salt.
• Analyze reactions: Check for insoluble byproduct or excess that requires filtration.
• Evaluate pairs: Determine if filtration step yields pure soluble salt solution.
• Select match: Choose option with excess insoluble reagent forming filterable impurity.

Why B is correct:

• Excess CuCO₃ reacts incompletely with HCl (CuCO₃ + 2HCl → CuCl₂ + H₂O + CO₂), leaving unreacted insoluble CuCO₃ as residue; filtration removes it, evaporation of CuCl₂ filtrate yields pure salt.

Why the others are wrong:

• A: Stoichiometric CuCO₃ + HCl fully dissolves to CuCl₂ solution; no insoluble residue, filtration unnecessary.
• C: AgNO₃(aq) + HCl(aq) → AgCl(s) + HNO₃(aq); filtration removes salt (AgCl), filtrate evaporation yields acid, not salt.
• D: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O; fully soluble, no residue, filtration irrelevant.

Final answer: B