Tests for Carbon Dioxide

Identifying Carbon Dioxide

Carbon dioxide (CO) is a gas commonly produced when carbon-containing compounds burn, such as fossil fuels or wood. It is colourless and odourless, making it invisible and hard to detect by smell or sight alone. Carbon dioxide is also denser than air, so it tends to settle in low areas.

Because it is produced in many combustion reactions and respiration, recognising carbon dioxide is important in chemical analysis and environmental monitoring.

Limewater Test for Carbon Dioxide

The most common and reliable test for carbon dioxide is the limewater test. Limewater is a clear, colourless solution of calcium hydroxide, Ca(OH).

When carbon dioxide gas is bubbled through limewater, the solution turns milky or cloudy. This happens because carbon dioxide reacts with calcium hydroxide to form calcium carbonate (CaCO3), which is insoluble and appears as a white precipitate:

$$\text{Ca(OH)}_2 (aq) + \text{CO}_2 (g) \rightarrow \text{CaCO}_3 (s) + \text{H}_2\text{O} (l)$$

This cloudiness confirms the presence of carbon dioxide.

If excess carbon dioxide is bubbled through, the calcium carbonate reacts further to form soluble calcium hydrogen carbonate, and the solution clears again:

$$\text{CaCO}_3 (s) + \text{CO}_2 (g) + \text{H}_2\text{O} (l) \rightarrow \text{Ca(HCO}_3)_2 (aq)$$

For instance, if you collect gas from a reaction where a carbonate reacts with an acid, bubbling this gas through limewater will turn it cloudy, confirming carbon dioxide is produced.

Other Confirmatory Tests for Carbon Dioxide

Besides the limewater test, carbon dioxide has other characteristic properties that can help confirm its identity:

• Non-flammable: Carbon dioxide does not support combustion. If you try to light a splint in a container of carbon dioxide, the flame will go out.
• Effect on moist blue litmus paper: Carbon dioxide dissolves in water to form a weak acid (carbonic acid), which turns moist blue litmus paper red and then eventually white due to acid damage. The red colour change is the primary indicator, while bleaching is a less common secondary effect.

These tests can be used alongside the limewater test to confirm the gas is carbon dioxide.

Learning Example

Imagine you collect a gas from the reaction of hydrochloric acid with calcium carbonate (marble chips). The gas produced is carbon dioxide. You bubble this gas through limewater, and the limewater turns milky. What gas is produced?

Since the limewater turns cloudy, this confirms the gas is carbon dioxide. This matches the expected reaction:

$$\text{CaCO}_3 (s) + 2\text{HCl} (aq) \rightarrow \text{CaCl}_2 (aq) + \text{CO}_2 (g) + \text{H}_2\text{O} (l)$$