Hydrogencarbonate indicator is a colourful way to measure carbon dioxide levels in aquatic ecosystems. It is red in equilibrium with atmospheric air, which then transforms into a orange/yellow colour with increased levels of carbon dioxide.

As carbon dioxide is removed, it changes from red through magenta to deep purple. The results seen in the pictures have been the best so far. Students were able to link these observations to what they knew about photosynthesis and indeed were able to confirm what you set out to show in the experiment.

The Science explained.
Green plants and algae use up carbon dioxide – removing it from the indicator turning it purple as they carry out photosynthesis.

BUT… they also produce carbon dioxide as they respire – all living things respire all the time, hence turning the indicator yellow. J. Yiangou

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