Design and make a device to lift as heavy an object as possible, at least 10 cm higher

As an energy source, the reaction between 1 level teaspoon of bicarbonate of soda (sodium hydrogencarbonate) and 3 level teaspoons of citric acid. Once lifted, the object must stay there.

This session could take around two hours and 30 minutes, or can be used for homework over a number of weeks.



  • Eye protection
  • Top-pan balance
  • Standard masses (10 g & 100 g slotted masses, and 1 kg masses for the ambitious)
  • Metre or half metre ruler
  • Identical teaspoons (can be plastic)


  • Sodium hydrogencarbonate (maximum amount = 3 level teaspoons)
  • Citric acid (maximum amount = 9 level teaspoons)
  • Water

Health, safety and technical notes

  • Read our standard health and safety guidance here
  • Wear eye protection.
  • Wear clothing protection if desired.
  • Citric acid is an eye irritant, see CLEAPSS Hazcard HC036c
  • This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.

Possible approaches

Perhaps guidance needed for younger age groups to say that water is needed for the reaction. The reaction might be used to do the lifting, or it could be used to start the lifting, eg to trigger movement of a counterbalance.

Possible extension 

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