A colorless solution contains a mixture of chloride (Cl⁻) and nitrate (NO₃⁻) ions. Which sequence of reagents will allow the chloride ion to be tested without interference from the nitrate ion?
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Correct answer
A. Add dilute nitric acid, then silver nitrate solution.
Principle or equation
Chloride ions are identified by adding silver nitrate solution, producing a white precipitate of silver chloride (AgCl) that is soluble in dilute ammonia but insoluble in dilute nitric acid. Nitrate ions do not interfere because they do not form a precipitate with silver nitrate. Dilute nitric acid is added first to remove other ions that might interfere (e.g., carbonate, sulfite) and to provide an acidic medium.
Why this answer is correct
Option A: Adding dilute nitric acid first removes carbonate, sulfite, etc., and then adding silver nitrate gives a white precipitate of AgCl if chloride is present. Nitrate ions do not react with silver nitrate, so no interference. Option B uses hydrochloric acid, which would introduce chloride ions, causing a false positive. Option C uses sodium hydroxide, which could cause precipitation of silver oxide, interfering. Option D uses sulfuric acid and lead(II) nitrate; lead chloride is sparingly soluble but not a standard test, and sulfate could interfere.
Example
To test for Cl⁻: add a few drops of dilute nitric acid, then silver nitrate solution. A white precipitate confirms Cl⁻.
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