Challenge pupils to balance sinking against floating, with a toy submarine concept

Design and make a device (with final dimensions NOT exceeding 15 cm × 15 cm × 15 cm) which will sink in water, and then after a reasonable length of time, rise to the surface.

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



  • A selection of junk materials
  • Plastic beakers, 100 cm3
  • Stop clocks
  • Masses, 10g 
  • Rubber bands
  • Rubber tubing (1 metre)
  • Sticky tape or masking tape
  • String
  • Identical teaspoons (plastic can be used)
  • Ruler, 30 cm 
  • Plastic buckets for ‘test runs’


For students who have studied neutralisation reactions:

  • Hydrochloric acid, 0.5 mol dm-3 
  • Marble chips
  • Tartaric acid

Selection of chemicals:

  • Salt
  • Sugar
  • ‘Alka Seltzer’ tablets
  • Andrews’ liver salts, or a mixture of solid sodium hydrogencarbonate/citric acid (1 teaspoon of sodium hydrogencarbonate to 3 teaspoons of citric acid).
  • Access to water. 

Health, safety and technical notes

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

Possible approaches

Generating a gas which causes a container to become buoyant; weighting a container with soluble material and allowing water to dissolve it so that the container then rises to the surface. (The sinking or floating/rising aspects could be investigated separately.) 

Possible extensions

Increasing the complexity of the behaviour required in terms of time limits and depth limits. Extend to actual lifting of a submerged object.