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

   * 8 stoppered large test tubes.
   * 4 pieces of pond weed
   * 4 water snails
   * Water
   * Some sort of carbon dioxide indicator such as bicarbonate indicator or
     bromothymol blue. Do not use anything toxic.

Method

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.

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