How does the air necessary for breathing enter the human body?

In the experimental setup to show that $CO_2$ is released during respiration, name the substance taken in the small test tube kept inside the conical flask. State its function and the consequence of its use.

What are respiratory substrates? Give one example.

In human beings, where does the gaseous exchange take place?

How does the exchange of gases occur in the roots of a plant?

If a person holds their breath after expiration for about $25 \ s$, would there be an occurrence of any exchange of respiratory gases in the lungs during this period? Explain.