A student is given three white solids: sodium chloride (NaCl), sodium carbonate (Na2CO3), and sodium sulfate (Na2SO4). Each is separately dissolved in water to form a 0.10 M solution. Which test would correctly distinguish sodium carbonate from the other two?
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Correct answer
A. Adding a few drops of dilute hydrochloric acid to each solution and observing for effervescence
Principle or equation
Carbonate salts react with dilute acids to produce carbon dioxide gas, which is observed as effervescence. Sodium chloride and sodium sulfate do not produce gas with dilute hydrochloric acid.
Why this answer is correct
When dilute hydrochloric acid is added to a carbonate, carbon dioxide gas is released, causing bubbling. Sodium chloride and sodium sulfate do not react with hydrochloric acid to produce a gas. Silver nitrate would precipitate chloride (AgCl) and carbonate (Ag2CO3), but not sulfate (Ag2SO4 is sparingly soluble). Barium chloride would precipitate sulfate (BaSO4) and carbonate (BaCO3), but not chloride. pH measurement would show carbonate solution is basic, but the question asks for a test that distinguishes carbonate from the other two; the acid test directly identifies carbonate by gas evolution.
Example
Add dilute HCl to a solution of Na2CO3: bubbles of CO2 are observed. No bubbles with NaCl or Na2SO4.
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