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TeacherExpt:Particles in motion?

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Students make a test tube of carbon dioxide by reacting hydrochloric acid with calcium carbonate. They then hold a test tube of air over the one of carbon dioxide and leave for a few minutes before testing both for carbon dioxide using limewater. They repeat the experiment, this time holding the one of carbon dioxide over the one of air. This shows that the gas diffuses between the two test tubes.


Lesson organisation

Any problems are most likely to occur while students are holding the test tubes and waiting for the gases to diffuse. You may like to have a video clip or something for them to watch during this time.

Apparatus and chemicals

  • Eye protection

Per pair or group of students:

  • Test tubes (3)
  • Delivery tube fitted to a test tube bung
  • Bungs to fit test tubes (3)
  • Access to:

Technical notes

Dilute hydrochloric acid (Low hazard at concentration used) Refer to CLEAPSS Hazcard 47 and Recipe card 31

Limewater (Low hazard) Refer to CLEAPSS Hazcard 18 and Recipe card 15

Calcium carbonate (Low hazard) Refer to CLEAPSS Hazcard 19B


A. Set up the apparatus as shown in the diagram.

B. Put a spatula of calcium carbonate into the first test tube. Add about 10 cm3 hydrochloric acid and quickly replace the bung and the delivery tube. Ensure that the delivery tube reaches almost to the bottom of the second test tube.

C. Carbon dioxide gas is made. Allow it to pass into the second test tube for about 1 minute and then remove the delivery tube and put a bung into the test tube. Put another test tube in its place and again remove it and put a bung in after about 1 minute.

D. Hold one of the test tubes upside down over a similar test tube containing air. Remove the bung and place the test tubes ‘mouth to mouth.’

E. After about 5 minutes put a bung in both tubes. Test them for carbon dioxide by dropping a little limewater into the tube and swirling it round. Record what happens in both tubes.

F. Take the second tube of carbon dioxide. This time hold the test tube of air upside down over the test tube of carbon dioxide and repeat step E.

Teaching notes

hydrochloric acid + calcium carbonatecarbon dioxide + water + calcium chloride

2HCl + CaCO3 → CO2 + H2O + CaCl2

The carbon dioxide is heavier than air and so might be expected to stay in the lower of the test tubes, however this is not the case – it moves into the upper tube too and the limewater goes milky in each test tube.

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

If the mixture of acid and calcium carbonate stops fizzing then it is no longer producing carbon dioxide. Just add a bit more acid.

If too much limewater is used then it may not turn milky. Only about 1 cm3 at the most is required.

Health & Safety checked, April 2008


This experiment has been adapted from Practical Chemistry: http://practicalchemistry.org/experiments/introductory/solids-liquids-and-gases/particles-in-motion,187,EX.html