Site author Richard Steane
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Experiments to compare carbon dioxide content of  inhaled and exhaled air

In this experiment, a volunteer breathes in through one tube containing a CO2-detecting liquid, and out through another tube containing the same CO2-detecting liquid.

The design of the apparatus should ensure that the same amount of air passes though each tube!

The liquids which can be used are either limewater or hydrogencarbonate indicator.

Limewater (calcium hydroxide solution) will turn cloudy/milky when a certain amount of carbon dioxide is passed through it.
It is a specific test for carbon dioxide.

On the other hand, carbon dioxide will cause hydrogencarbonate indicator to turn from a reddish colour to an orange/yellow.
It is actually not a specific test for carbon dioxide, relying on small pH changes due to carbonic acid, formed when carbon dioxide reacts with water.

Experimental details

Use the mouse, or tap the screen, to show which way the air moves

Check that the apparatus is assembled correctly:

The T-tube section is connected at one side to the long delivery tube dipping into one boiling tube, and at the other side to the short delivery tube dipping into the other boiling tube
Both boiling tubes contain about the same amount of liquid, and the tip of each long delivery tube is at about the same level in the liquid.
Bungs are fairly securely inserted.

Breathe in very gently. Note that air bubbles in via the left boiling tube, as indicated on the diagram.

Without removing your mouth from the apparatus, breathe out gently. Note that air bubbles out via the other boiling tube.

Continue to breathe in and out without removing the tube from the mouth. This should ensure that the the same amount of air passes through both tubes.


Use the mouse, or tap the screen, to reveal the results.
        Limewater         Hydrogencarbonate

apparatus set up containing limewater out  


apparatus setup for use with hydrogencarbonate indicator

The air we breathe in contains about 0.04% carbon dioxide.
The air we breathe out contains about 4% carbon dioxide.
In other words, exhaled air contains about 100 times the concentration of carbon dioxide that inhaled air does.
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