Give two reasons to explain why air is a mixture and not a compound.

Explain how the percentage by volume of oxygen and carbon dioxide in the air is maintained at about $21\%$ and $0.03\%$ respectively.

Air contains nitrogen and oxygen. Name two other gases present in the air.

What are combustible and non-combustible substances?

Name two substances other than oxygen that support combustion.

Compare the burning of carbon compounds with respiration. How is respiration similar to burning?

Name the gases added to air by:

Combustion

Name the gases added to the air by:

Decay or fermentation

Name the gases added to the air by:

Photosynthesis.