Experiment to compare oxygen and carbon dioxide content of inhaled and exhaled air

2. Lower a burning candle quickly into one jar of inhaled air.
3. Close the jar and record how long the candle can burn.
4. Repeat with one jar of exhaled air.

5. Add 10cm3 of hydrogencarbonate indicator into one jar of inhaled air.
6. Close the jar and shake.
7.  Observe any colour change.
8. Repeat with one jar of exhaled air.

7) Result

8) Discussion
a. Interpret result
 Results show that inhaled air contains more oxygen but less carbon dioxide than  
 exhaled air.
 b. Source of error
 (i)   The size of the flame may have different and it may effect the time burning
 (ii)  The starting and the ending time is different and it may effect the time we use in
        the experiment and effect the accuracy of measuring the time.

  (iii) There may have some water droplets in the inhaled gas jar and it may effect the  
        content of carbon dioxide.
c. Improvement
 (i)   Use a more similar candle or simply use the same candle
 (ii)  Establish when we start and stop timing
d. Further investigation
 We may also find that the temperature of exhaled air is higher than that of inhaled air
 when we breathe onto our hands. Exhaled air is warmed by the blood in the
 capillaries.

9) Conclusion
Oxygen in inhaled air diffuses from the air sacs into blood and is then used by body cells for respiration. Carbon dioxide produced by body cells during respiration diffuses from the blood into the air sacs and is then exhaled. We can find out the  oxygen and carbon dioxide content of inhaled and exhaled air in this experiment. Some students wrongly think that inhaled air consists mainly of oxygen, and exhaled air contains mainly of carbon dioxide. In face, both inhaled and exhaled air consists mainly of nitrogen ( about 78%). Some students wrongly think that there is more carbon dioxide than oxygen in exhaled air. Though exhaled air contains a higher proportion of carbon dioxide when compared with that in inhaled air, its absolute amount( in terms of percentage) is still lower than that of oxygen.