How good are you at identifying gasses in the classroom…

In the beaker are test-tubes containing different gases - carbon dioxide, dinitrogen oxide, oxygen, chlorine and hydrogen. You may remove a test tube only once, and when you do so, you must identify the gas immediately.


  • Lime water
  • Litmus paper
  • Spills

Materials per group

Five test-tubes of each of the gases:

  • Carbon dioxide
  • Hydrogen
  • Oxygen
  • Chlorine
  • Dinitrogen oxide

The test tubes must be inverted, and the tops must be under the level of the water.

Equipment per group

  • Beaker filled with water
  • Bunsen burner
  • Heat-resistant mats
  • Safety glasses

Health, safety and technical notes

  • Read our standard health and safety guidance here.
  • Wear eye protection. 
  • This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.


During trialling, this was set as both a practical and a theoretical problem. Students looked up the reactions and properties of gases that they were unfamiliar with in order to solve the problem. Swirling the test-tubes should cause one water level to rise (N2O).

Close examination should yield the yellow colour of chlorine, or a piece of litmus paper held near the bottom of the test-tube should turn red then turn colourless due to bleaching. Removal of the test-tube containing the carbon dioxide and its immersion in a beaker of lime water should give a cloudy solution.

By keeping the remaining test-tubes inverted after removing them from the water, there should be enough time to carry out the tests for hydrogen and oxygen.


The experiment can be modified by varying the gases in the test-tubes – e.g. by using nitrogen monoxide and nitrogen dioxide.


This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources, and obtain a list of suggested ‘junk items’ here.