Testing for carbon dioxide: limewater turns cloudy
Definition
Carbon dioxide
A gas made from carbon and oxygen, with one carbon atom for every two oxygen atoms.
Definition
Limewater
A colourless solution of calcium hydroxide used to test for carbon dioxide; the gas turns it cloudy white.
- Bubble the gas through limewater, which is a solution of calcium hydroxide.
- If carbon dioxide is present, the limewater turns cloudy (milky) white.
- The cloudiness is fine particles of insoluble calcium carbonate: Ca(OH)2+CO2→CaCO3+H2O\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}Ca(OH)2+CO2→CaCO3+H2O.
- Bubbling the gas through several times makes the change easier to see.
Exam technique
- Say the limewater turns cloudy or milky, not just that it changes.
- Name calcium carbonate as the solid to earn the explanation mark.
Why it works and common slips
- Limewater starts as a clear, colourless solution; the milkiness is solid calcium carbonate forming.
- Give the reaction time, because a small amount of gas may need several bubbles to show clearly.
- Do not say the limewater turns solid white; it turns cloudy, a suspension of fine particles.
Self review
- How do you test for carbon dioxide?
- What do you see if carbon dioxide is present?
- Name the white solid that makes limewater cloudy.
- Write the equation for carbon dioxide reacting with limewater.
