A white solid is known to be either magnesium oxide (MgO) or calcium carbonate (CaCO3). Which test would best distinguish between them?
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Correct answer
A. Add dilute hydrochloric acid and observe for gas bubbles
Principle or equation
Carbonates react with dilute acids to produce carbon dioxide gas, which bubbles. MgO reacts with acid to form a salt and water without gas evolution.
Why this answer is correct
Adding dilute HCl to CaCO3 produces CO2 bubbles, while MgO dissolves without gas. Electrical conductivity of solids is negligible for both. Both oxides produce basic solutions, but CaCO3 is sparingly soluble. Heating CaCO3 decomposes to CaO and CO2, but this is a valid test; however, the acid test is simpler and more direct.
Example
If a sample is CaCO3, adding HCl yields effervescence: CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g).
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