Question

Why is air sometimes considered as a heterogeneous mixture?

Answer 1

We have to know that the air is the one of the important things in our life. In the world, without air nothing is surveyed. For human beings, breathing purpose air is the important thing for inhaling and exhaling. For human respiration, oxygen is used for inhalation of breathing and carbon dioxide is exhaled gas. This inhale and exhale gas are important for human lifestyle to survey in the world. Each and every gas has unique properties and unique nature in the atmosphere. One gas is not able to replace another gas in the environment.

Complete answer:
We must have to know that the air is nothing but a mixture of gases. The mixture of the gas in the air are major in the form of nitrogen, oxygen, carbon dioxide and other gases.
In the atmosphere approximately $78\%$ of nitrogen, $21\%$ of oxygen and remaining $1\%$of other gases in the world.
Sometimes, one of the gas in the air in the form of liquid depends on temperature. This kind of biphasic nature of gas will happen.
That is the reason, air is sometimes considered as a heterogeneous mixture.

Note:
We must remember that the chemical properties of the gas depend on the mixture of gases. All the gases are not helpful for the human body. Some gases are poison for the human body. It leads to death also. For example, wrong inhalation of carbon dioxide in the human body leads to death. Some cyanide gases and mono and dioxide gases also lead to human death in the world. In plants during the day time they inhale carbon dioxide from the atmosphere and exhale oxygen, but in night time they inhale oxygen and exhale carbon dioxide.