To identify the presence of carbonates in a sample, follow these steps:
1. First, add dilute acid to the sample.
2. If there are carbonates present, the acid will react with the carbonate to form carbon dioxide gas, which you will see as effervescence (fizzing).
CO3²⁻ (aq) + 2H⁺ (aq) → CO2 (g) + H2O (l)
3. Bubble the gas through limewater.
The carbon dioxide gas reacts with limewater in the following reaction:
CO2 (g) + Ca(OH)2 (aq) → CaCO3(s) + H2O (l)
If the limewater turns cloudy, then the gas is carbon dioxide. Carbonates react with dilute acids to form carbon dioxide, which means that we originally started with a carbonate ion.
Let’s look at an example in which we add calcium carbonate to hydrochloric acid.