Warning! Warning! Science in progress!

Create a warning system using junk materials and chemistry knowledge.


It is suggested that either: an entire morning be devoted to the problem (eg on the last day of term), which would allow 2 hours for practical activities and 30 minutes for judging


the problem be given to the class as a homework exercise 2 weeks or so before the judging. Judging could then take place in a normal double science lesson, allowing 45 minutes for repair and final adjustments, and 30 minutes for judging. 


  • Items from the junk list, for example balloons, surgical gloves – should be chosen to encourage creativity.

Materials per group

  • Sodium hydrogencarbonate (maximum amount = 3 level teaspoons)
  • Citric acid (maximum amount = 9 level teaspoons)
  • Access to water

Equipment per group

  • Identical teaspoons (can be plastic)
  • 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.
  • Citric acid is an eye irritant, see CLEAPSS Hazcard HC036c


Guidance may be needed for younger age groups to say that water is needed for the reaction. The reaction might be used to do the moving, or it could be used to start the movement – e.g. to trigger the movement of a counterbalance to light a bulb or rattle a tin. During trialling, some students inflated a surgical glove, while others used the reaction to complete an electric circuit and thus rung a bell. A variety of approaches were seen – some more elegant than others! 


To increase the chemical content, the task could be extended by prior (or subsequent) experimentation, to select the best choice of gases/chemicals. 


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.