Showing carbon dioxide comes out
That carbon dioxide is released in respiration can be shown experimentally.
Activity 02
Connect five vessels in a row, A to E. Slowly draw the water out of bottle E and a stream of air flows from A through to E.
| Vessel | Contains | Its job |
|---|---|---|
| A | Potassium hydroxide | Removes CO2 from the incoming air |
| B | Lime water | Checks that A did its job |
| C | A frog | The organism doing the respiring |
| D | Lime water | Tests the outgoing air for CO2 |
| E | Water | The suction that creates the air stream |
Figure to be added
Observation
The lime water in B does not change colour. The carbon dioxide in the air entering at P dissolves in the KOH in vessel A, so no CO2 reaches B.
After a while the lime water in D turns milky. The reason is that the frog in vessel C has been respiring and giving out carbon dioxide.
Why is B there at all?
Without B, when the lime water in D turned milky there would be no telling whether that CO2 came from the frog or was already in the air that entered. B proves the incoming air carries no CO2 — which leaves the frog as the only explanation for what happens in D.
The control
A setup with no frog in vessel C can be used as the control experiment — the lime water in D should then not change. The experiment also works with a good quantity of germinating seeds — green gram, cowpea, paddy or maize — in place of the frog in C.