Reviewed CSCA Chemistry question · Standard

A white solid is known to be either magnesium oxide (MgO) or magnesium carbonate (MgCO3). Which test would best distinguish between them?

Add dilute hydrochloric acid and look for effervescence.
Add water and test the pH of the resulting solution.
Heat the solid strongly and test any gas evolved with limewater.
Add silver nitrate solution and look for a precipitate.

Show the answer and explanation

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 acids.

Why this answer is correct

MgCO3 reacts with dilute HCl: MgCO3(s) + 2HCl(aq) -> MgCl2(aq) + CO2(g) + H2O(l). Bubbles of CO2 are observed. MgO reacts with acid to form MgCl2 and water, but no gas is produced.

Example

Add a few drops of dilute HCl to each solid. The one that fizzes is MgCO3.

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