A white powder is known to be either calcium oxide (CaO) or calcium carbonate (CaCO3). Which test would best distinguish between them?
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Correct answer
A. Add dilute hydrochloric acid and look for effervescence.
Principle or equation
Carbonates react with acids to produce carbon dioxide gas, which causes effervescence. Oxides do not produce gas with acid.
Why this answer is correct
Calcium carbonate reacts with dilute HCl to produce CO2 gas, seen as bubbles. Calcium oxide reacts with acid to form a salt and water, with no gas. Thus, adding dilute HCl and observing effervescence distinguishes them. Option B is not definitive because both can give basic solutions. Option C would produce CO2 from carbonate but also from oxide? Actually CaO does not contain carbonate, heating does not produce CO2. But option A is simpler and more direct. Option D is not relevant.
Example
Add a few drops of dilute hydrochloric acid to a small sample of the powder. If bubbles are observed, the powder is CaCO3; if no bubbles, it is CaO.
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