HKCEE MC

The answer is : C. acidified silver nitrate solution

A. is false.
When sulphuric acid is added to each of the three solutions :
Sodium carbonate solution gives colourless bubbles, because sodium carbonate reactswith dilute sulphuric acid to give carbon dioxide gas.
2H^+(aq) + CO3^-(aq) → H2O(l) + CO2(g)
Both potassium chloride solution and dilute nitric acid have no reaction andthus give no observable change. Therefore,dilute sulphuric acid CANNOT be used to distinguish them.

B. is false.
When sodium hydroxide solution is added to each of the three solutions :
Dilute nitric acid gives off heat, because the neutralization of dilute nitricacid with sodium hydroxide is exothermic.
H^+(aq) + OH^-(aq) → H2O(l) .. (exothermic)
Both sodium carbonate solution and potassium chloride solution have no reactionand thus give no observable change. Therefore, sodium hydroxide solution CANNOT be used to distinguish them.


C. is true.
When acidified silver nitrate solution is added to each of the three solutions:
Sodium carbonate solution gives colourless bubbles, because sodium carbonate reactswith the hydrogen ions in the acidified solution to give carbon dioxide gas.
2H^+(aq) + CO3^-(aq) → H2O(l) + CO2(g)
Potassium chloride gives a white precipitate, because silver chlorideprecipitate is formed.
Ag^+(aq) + Cl^-(aq) → AgCl(s)
Dilute nitric acid has no reaction and thus gives no observable change.
Therefore, acidified silver nitrate solution can be used to distinguish them.

D. is false.
When acidified potassium dichromate solution is added to each of the three solutions:
Sodium carbonate solution gives colourless bubbles, because sodium carbonate reactswith the hydrogen ions in the acidified solution to give carbon dioxide gas.
2H^+(aq) + CO3^-(aq) → H2O(l) + CO2(g)
Both potassium chloride solution and dilute nitric acid have no reaction andthus give no observable change. Therefore,acidified potassium dichromate solution CANNOT be used to distinguish them.

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