A few drops of concentrated HNO 3 is added to an unknown solution and boiled for a while. If this produces a…

Analyze the question: Concentrated HNO₃ is a strong oxidizing agent. When added to a solution and boiled, a brown solution forms. We need to find which cation fits this observation.

Concept: Fe²⁺ (pale green) is oxidized by hot concentrated HNO₃ to Fe³⁺, which appears yellow-brown/brown in solution. This is a standard test used to confirm Fe²⁺ (the nitric acid oxidation step is followed by NaOH or NH₃ to give a reddish-brown precipitate of Fe(OH)₃).

Check each option:

• A. Pb²⁺: Already in its stable oxidation state and colourless. HNO₃ causes no colour change.
• B. Cu²⁺: Already Cu²⁺ (blue). It cannot be oxidized further by HNO₃, so no brown solution forms.
• C. Fe³⁺: Already Fe³⁺, so HNO₃ would not change it. It is already yellow-brown, so no change is "produced" by the acid.
• D. Fe²⁺: Oxidized to Fe³⁺ by hot HNO₃, producing the brown solution.

Equation: 3Fe²⁺ + NO₃⁻ + 4H⁺ → 3Fe³⁺ + NO + 2H₂O

(The NO released may further react with air to give brown NO₂ fumes, and the Fe³⁺ gives the brown colour to the solution.)

Answer: The cation is Fe²⁺, which is option D.

Exam tip: "Concentrated HNO₃ + boil" in cation tests means oxidation. Only a cation that can be oxidized (Fe²⁺ → Fe³⁺) shows a change. Remember: Fe²⁺ is green, and after the HNO₃ test it turns yellow-brown as Fe³⁺.