Test for carbon dioxide: The test for carbon dioxide uses an aqueous solution of calcium hydroxide (lime water). When carbon dioxide is shaken with or bubbled through limewater the limewater turns milky (cloudy).

The idea

Carbon dioxide is detected using limewater, which is an aqueous solution of calcium hydroxide, Ca(OH)₂(aq). 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.

What earns the marks

• 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.

Common mistakes and how to put them right

• Saying the limewater turns white rather than milky or cloudy.

  Put it right: 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.

  Put it right: 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.

  Put it right: Limewater is calcium hydroxide solution, whereas limestone is mainly calcium carbonate, so the reagent and product must not be swapped.

Exam 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.