1. Carbon Dioxide from Respiration
Carbon dioxide from respiration is demonstrated by two chemical reactions

2. PURPOSE
To demonstrate that carbon dioxide is expelled from the lungs by seeing it react with limewater (calcium hydroxide) and bromothymol blue indicator

3. DEMONSTRATION:
When carbon dioxide is blown through a straw into a flask of limewater (calcium hydroxide) the carbon dioxide reacts with the solution to produce insoluble calcium carbonate. When the same is done with bromothymol blue indicator, the carbon dioxide turns the solution from blue to yellow-green.

4. CONCEPTS:
Respiration
Solubility pH
Indicators

5. RESPIRATION
Respiration is the ventilation of the lungs (breathing) and the exchange of gases between blood and the atmosphere. The exchange of gases occurs in the alveoli of the lungs in which oxygen is loading and carbon dioxide is unloading. In this demonstration, the carbon dioxide used for chemical changes was produced by respiration.

6. SOLUBILITY
The solubility of a substance is the amount of that substance that can be dissolved in a given quantity of solvent. In this demonstration the reaction of the carbon dioxide and limewater produced a fine, white precipitate of calcium carbonate. The reason a precipitate formed is because calcium carbonate is insoluble in water. Calcium carbonate is limestone (chalk is a finely-grained calcium carbonate).
CO2(g) + Ca(OH)2(l) CaCO3(s) + H2O(l)

7. pH and INDICATORS
Bromothymol blue is an indicator that turns yellow-green in the presence of acidic substances. When carbon dioxide is breathed into water it reacts with the water to form carbonic acid.
CO H2O H2CO3
Carbonic acid itself is in equilibrium with water and carbon dioxide.
As more and more carbonic acid is produced, the pH of solution decreases causing the indicator to change color to yellow-green. The color change of the indicator thus serves to demonstrate the presence of carbon dioxide by showing the change in pH.

8. CONCLUSION
The carbon dioxide produced from respiration was used to do chemical reactions which demonstrated its presence. The reaction of carbon dioxide and calcium hydroxide produced calcium carbonate which was insoluble in water and formed a white precipitate.
As the carbon dioxide reacted with the water in the bromothymol blue indicator, the pH decreased (became more acidic) causing the blue color to change to yellow-green.

9. COMMENTS
You can also use dry ice with the same effects for the source of carbon dioxide in this experiment,

10. Fun Facts
Carbon dioxide does not burn or support combustion. Because of this property, it is used in some fire extinguishers.