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Simple Gas Diffusion


Students make carbon dioxide and then observe it mix with air.


Students firstly make a small amount of carbon dioxide by mixing dilute hydrochloric acid and calcium carbonate in a test tube. They then take the tube of CO2 and hold it mouth-to-mouth with another tube of air (an empty tube). After a period of about 5-10 minutes, each tube is tested for the presence of CO2 by adding a drop of limewater to each tube and shaking it gently.

Equipment required (per set):

  • test tubes (3)
  • lime water
  • spatula
  • dilute HCl (0.4mol dm3 - low hazard) approx 10cm3
  • calcium carbonate chips (low hazard) small spatula
  • delivery tube with bung attached ('n' shape)
Only a small amount of calcium carbonate and acid is needed for this practical. Students should be told not to overfill the test tubes.

By holding one tube of gas mouth-to-mouth with another, over time the gases contained within each will diffuse into each other. In this practical, students should, with the limewater test, see that both tubes contain a quantity of CO2 when given enough time to diffuse.

Even though the carbon dioxide gas is heavier than the air in the other tube, it will over time diffuse into the upper tube. This shows that the gas particles are constantly in motion.

The formula for the reaction is below:

2HCl + CaCO3 → CO2 + H2O + CaCl2


 CautionWear eye protection.

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