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Using water snails and pond weed to investigate the carbon cycle

All livings things respire. In addition plants photosynthesise given the correct conditions. The following experiment demonstrates the balance of carbon dioxide (CO2) in a closed ecosystem.

You will need


Label the test tubes 1 to 8. Add a small amount of indicator to all of them. Fill them with water.

  • In tubes 1 and 5 Add a water snail
  • In tubes 2 and 6 Add a piece of pond weed
  • In tubes 3 and 7 Add a snail and pond weed
  • In tubes 4 and 8 Don't add anything. These will be controls

Stopper all the tubes then put tubes 1 to 4 in a brightly lit position, and tubes 5 to 8 in the dark. Leave for the shortest possible time that it takes for the indicator to change colour. If you leave it for more than about 30 minutes, you will kill the snails.

After the allotted time the tubes that were kept in the dark will all become acidic because of excess carbon dioxide (with the exception of tube 8; the control tube). This is because respiration gives off CO2 as one of the products, and carbon dioxide dissolves in water to make an acidic soloution.

The brightly lit tubes will show excess CO2 in tube 1 but reduced CO2 in tube 2 because photosynthesis uses up carbon dioxide. Tube 3's exact CO2 content is difficult to predict because it depends on the size of the snail, amount of pondweed and the brightness of the light. It should however be closer to the control than either tubes 1 or 2.

See Also School Biology experiments and demonstrations

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