Theory:

This can be proved by a small activity:
 
Materials required:
  • Two glasses
  • Lime water
  • Straw
Procedure:
  • Take two test tubes and fill them with lime water.
  • Leave the glasses undisturbed.
  • With the help of a straw blow air into the second tube and observe what happens by recalling chapter 6 (Physical and chemical changes)
 
Observation:
 
The lime water in the second tube turns milky white.
 
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Inference:
  
We know that we inhale a mixture of gases. But from this activity we can say that we exhale carbon diode. This is because the lime water present in the second tube turns milky after a few seconds due to the presence of carbon dioxide in the air that we exhale. Hence we can say that carbon dioxide is present in the air that we exhale.