Three separate colourless solutions each contain one cation, Na⁺, Pb²⁺ or Ca²⁺ - HSC - SSCE Chemistry - Question 8 - 2009 - Paper 1

We need to consider each reagent's response with Pb²⁺:

• KI: Reaction produces a precipitate of PbI₂ indicating the presence of lead ions.
• K₂CO₃: Produces a carbonate precipitate but not specific to lead.
• K₂PO₄: Forms a precipitate of Pb₃(PO₄)₂ but may also react with other metals.
• AgNO₃: Produces a white precipitate of PbCl₂ upon reaction with Pb²⁺, but it is not as specific.

Among these, KI is the best option as the precipitate can be obviously identified as lead iodide (PbI₂).

Given the analysis above, the most appropriate reagent to unambiguously identify the solution containing Pb²⁺ is (A) KI.