The limewater test
There are two tests for carbon dioxide — a flame going out, and limewater turning milky.
Activity 16.10
- Set up the apparatus of figure 16.5 and collect CO2 gas into several test tubes.
- Light a dry splint and, with the flame still burning, insert it into a test tube containing CO2.
- Carefully dissolve a little slaked lime {Ca(OH)2} in about 50 ml of water and filter it through filter paper. Put about 5 ml of it into a tube of CO2, stopper it firmly and shake well. Put the same amount into a tube of ordinary air, shake it, and compare the colour of the solutions in the two tubes.
The observations
The flame goes out at once. Not only that: the glowing ember on the splint soon goes out too.
It is the tube that held CO2 in which the limewater turns milky.
Why does it turn milky?
The calcium hydroxide {Ca(OH)2} in the limewater reacts with the carbon dioxide gas in the tube.
Ca(OH)2 + CO2 → CaCO3 + H2O
A suspension gives the colour
The limewater turns milky because the white calcium carbonate (CaCO3) formed is suspended in the water.
What if you keep going?
Pass more carbon dioxide into the tube holding that suspension and the gas reacts with the calcium carbonate to form calcium bicarbonate {Ca(HCO3)2}, which is soluble in water. The milkiness then disappears — so the test must be read promptly.