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Grades 8 and 10 Intermediate About 15 min Version 1 Awaiting review Premium

Testing for carbon dioxide with limewater

Carbon dioxide is a colourless gas with no smell, so you cannot see it. Chemists test for it with limewater, a clear solution of calcium hydroxide. When carbon dioxide bubbles through limewater, it reacts to form tiny particles of chalk (calcium carbonate), which make the limewater milky. In the laboratory, carbon dioxide is made by adding dilute hydrochloric acid to marble chips.

What you will learn

  • Describe how carbon dioxide is prepared from marble chips and dilute acid.
  • Use the limewater test to identify carbon dioxide.
  • Show that the air we breathe out contains more carbon dioxide than fresh air.

Before you start

  • Know that bubbles of gas can be evidence of a chemical reaction.

Materials

  • Marble chips (calcium carbonate)
  • Dilute hydrochloric acid
  • Limewater
  • Drinking straw and air pump

Equipment

  • Conical flask with a delivery tube
  • 3 test tubes in a rack

Procedure

  1. Add dilute hydrochloric acid to the marble chips in the flask to make carbon dioxide.
  2. Bubble carbon dioxide from the flask through the limewater in Tube A.
  3. Breathe out gently through a straw into the limewater in Tube B.
  4. Pump fresh air through the limewater in Tube C.
  5. Compare the three tubes.

How this simulation is simplified

  • Fresh air contains a little carbon dioxide (about 0.04%), too little to turn limewater milky in a short test.
  • Bubbling gas does not change the liquid volume.