Group 17: Question 10

Worked solution

Step 1: Recall the purpose of adding the dilute acid

Aqueous silver nitrate does not react only with halide ions. It also forms precipitates with other anions that might be present, such as carbonate ($\text{Ag}_2\text{CO}_3$) or hydroxide ($\text{AgOH}$). Adding a dilute acid first removes these interfering ions (e.g. carbonate is converted to $\text{CO}_2$ and water), so that only a genuine halide will produce a precipitate with the silver nitrate added afterwards.

Step 2: Identify why dilute nitric acid specifically is chosen

The acid added for this purpose must not introduce any new ion that could itself react with silver ions. Nitric acid, $\text{HNO}_3$, is ideal because its anion, $\text{NO}_3^-$, does not form a precipitate with $\text{Ag}^+$. Silver nitrate itself is soluble.

Step 3: Explain why hydrochloric acid is unsuitable

Hydrochloric acid, $\text{HCl}$, contains chloride ions. If it were added to the unknown solution before the silver nitrate test, it would introduce extra $\text{Cl}^-$ ions of its own. These would then react with the silver nitrate to form a white precipitate of silver chloride:

$\text{Ag}^+\text{(aq)} + \text{Cl}^-\text{(aq)} \rightarrow \text{AgCl(s)}$

This precipitate would appear regardless of which halide (if any) was originally present in the unknown solution, completely undermining the purpose of the test. Hydrochloric acid does remove carbonate and hydroxide ions just as well as nitric acid does. Its problem is specifically that it contaminates the sample with chloride ions.

Why the other options are wrong

• A: hydrochloric acid’s unsuitability is not because it is “too strong” or reacts with the halide already present (halide ions do not react with $\text{H}^+$); the real issue is the chloride ions it introduces.
• C: hydrochloric acid removes carbonate/hydroxide ions perfectly well; this is not why it is unsuitable.
• D: the two acids are not interchangeable. Hydrochloric acid’s chloride ions would contaminate the test, giving a false result.

Final answer

• Dilute nitric acid should be used; dilute hydrochloric acid is unsuitable because it would introduce chloride ions of its own, which would form a silver chloride precipitate and give a false result, option B.