Generate carbon dioxide from a chemical reaction and perform the limewater test to show the mix of gases

This experiment provides evidence that particles in a gas are in motion.

This experiment should take 30 minutes.



  • Eye protection
  • Test tubes, x3
  • Cork
  • Delivery tube and bung


  • Limewater 0.02 mol dm–3
  • Calcium carbonate
  • Hydrochloric acid 0.5 mol dm–3

Health, safety and technical notes


Procedure diagram

  1. Set up the apparatus as shown in the diagram.
  2. Put a spatula measure of calcium carbonate into the first test-tube. 
  3. Add 10 cm3 of hydrochloric acid and quickly replace the bung and delivery tube. Ensure the delivery tube reaches almost to the bottom of the second test-tube.
  4. Allow the gas to pass into the second test-tube for about one minute, then remove the delivery tube and cork the test-tube.
  5. Hold the test-tube upside down over a similar test-tube containing air.
  6. Remove the cork and place the tubes mouth-to-mouth.
  7. After 5 min, cork both tubes and test the contents for carbon dioxide (swirl a little limewater round in the test-tube). Write down what happens in both tubes.
  8. Repeat this experiment but this time at step 5 hold the test-tube of air upside down over a test-tube of carbon dioxide.


This experiment provides a good introduction, one suggestion is to show a demonstration of Brownian motion using a smoke cell after this experiment

Solids, liquids and gases consist of minute particles. If this were not the case, they would not mix so easily.

This is not proof of a particulate theory, but the experiment does suggest that the particles in the gas must be in motion to spread through the air in the containers.


  1. Which of the four test-tubes contained carbon dioxide at the end of the experiment?
  2. Is air or carbon dioxide more dense?
  3. Does this experiment support the idea that the particles of a gas are in motion? Give your reasons


  1. All the test-tubes contained carbon dioxide; the gases always diffuse and mix.
  2. Carbon dioxide is denser than air.
  3. Yes; both tubes should give cloudy limewater, suggesting the gases in the two tubes mixed. Some of the heavier carbon dioxide molecules moved upwards into the test-tube containing air.