When carbon dioxide is shaken with or bubbled through limewater the…

The idea

This statement describes the positive result of the carbon dioxide test. Carbon dioxide is either shaken with limewater in a sealed container or bubbled through it. The limewater, which is aqueous calcium hydroxide, turns milky or cloudy. The cloudiness is caused by solid calcium carbonate forming as tiny particles suspended in the liquid. The word milky is accepted as equivalent to cloudy. The change is from a clear solution to a cloudy one, so the starting limewater must be clear. A negative result leaves the limewater clear. This observation confirms carbon dioxide, for example from a reaction or from a gas sample.

What earns the marks

• State that carbon dioxide is shaken with limewater or bubbled through limewater.
• Describe the positive observation as the limewater turning milky or cloudy.
• Explain that the cloudiness is due to calcium carbonate forming as a solid precipitate.
• Note that the limewater starts clear, so the change to cloudy is the key evidence.
• Recognise that milky and cloudy describe the same observation.

Common mistakes and how to put them right

• Saying the limewater turns white or solid.

  Put it right: The correct observation is that it turns milky or cloudy as fine particles form.

• Claiming the limewater stays clear when carbon dioxide is present.

  Put it right: The correct result is cloudiness, while clear limewater indicates no carbon dioxide.

• Describing a colour change to a different colour.

  Put it right: The correct change is from clear to milky or cloudy, not to a coloured solution.

Exam tips

• Use the phrase 'turns milky' or 'turns cloudy' exactly, as both are accepted observations.
• Link the cloudiness to calcium carbonate forming, showing understanding rather than just recall.
• State the method briefly: bubble the gas through limewater or shake the gas with limewater.