Test for carbon dioxide — AQA GCSE Combined Science
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Test for carbon dioxide explained
Carbon dioxide is detected using limewater, which is an aqueous solution of calcium hydroxide, Ca(OH)₂(aq).
Read the full explanation
The gas is either bubbled through the limewater using a delivery tube or shaken with it in a stoppered container so the gas dissolves and reacts. Carbon dioxide reacts with calcium hydroxide to form solid calcium carbonate, which appears as a fine white precipitate, so the solution turns milky or cloudy. The test is positive only when this cloudiness appears; a colour change alone is not the observation. For example, bubbling exhaled breath through limewater turns it milky, showing that respiration releases carbon dioxide. The method matters: the limewater must be clear at the start, and enough gas must be passed to produce visible cloudiness.
Your focus
- Identify limewater as aqueous calcium hydroxide and describe how carbon dioxide is brought into contact with it.
- Describe the milky or cloudy appearance produced when carbon dioxide reacts with limewater.
- Explain that the cloudiness is caused by insoluble calcium carbonate forming as a white precipitate.
Test for carbon dioxide exam tips
Marking Points
- Limewater is an aqueous solution of calcium hydroxide, Ca(OH)₂(aq), and is the reagent used to test for carbon dioxide.
- The gas is bubbled through the limewater or shaken with it so that carbon dioxide comes into contact with the solution.
- Carbon dioxide reacts with calcium hydroxide to produce calcium carbonate, CaCO₃, which is insoluble and forms a white precipitate.
- The positive result is that the limewater turns milky or cloudy; this cloudiness is caused by the suspended calcium carbonate.
- A clear limewater result after adding gas is a negative result, so the test must be repeated with more gas or a fresh sample if no cloudiness appears.
Examiner Tips
- 💡State the reagent by its chemical name and formula, calcium hydroxide, Ca(OH)₂(aq), as well as the common name limewater.
- 💡Describe both the method and the result: gas bubbled through or shaken with limewater, and limewater turns milky or cloudy.
- 💡Link the observation to the product by naming calcium carbonate as the white precipitate responsible for the cloudiness.
Common Mistakes
- Saying the limewater turns white rather than milky or cloudy: the solid is a fine white precipitate suspended in the liquid, so the correct observation is cloudiness, not a solid white lump.
- Writing that carbon dioxide turns limewater clear: clear describes the starting limewater, while the positive test makes it cloudy, so the direction of the change must be stated correctly.
- Confusing limewater with lime or limestone: limewater is calcium hydroxide solution, whereas limestone is mainly calcium carbonate, so the reagent and product must not be swapped.