Testing for Carbonate Ions

To identify the presence of carbonates in a sample, follow these steps:

1. First, add dilute acid to the sample.

  • Any acid will work, but dilute hydrochloric acid is typically used

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) → CO(g) + H2O (l)

3. Bubble the gas through limewater.

The carbon dioxide gas reacts with limewater in the following reaction:

CO(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.

A detailed scientific illustration showcasing a laboratory experiment. On the left, a test tube is held in place by a stand. Inside the test tube, there are pieces of calcium carbonate immersed in hydrochloric acid. A rubber stopper seals the top, and attached to it is a delivery tube that redirects the gas released from the reaction into another test tube on the right, which is held by a hand and contains limewater. As carbon dioxide bubbles from the left tube pass through the limewater in the right tube, they create a milky cloudy white precipitate, indicating the presence of carbonate ions. Below the illustration, there's a description: "Adding calcium carbonate to hydrochloric acid will produce calcium chloride, carbon dioxide, and water," accompanied by the chemical equation "CaCO3 + 2HCl → CaCl2 + CO2 + H2O."