Air is a mixture of gases. The following are the percentage composition of the constituents of air: Nitrogen (78%), Oxygen (21%), Carbon (IV) oxide (0.03%), Noble gases (1%), Water vapor and impurities (variable).

Air is classified as a mixture due to the following reasons

1. The composition of air cannot be represented by a chemical formula
2. The constituents of air can be separated by physical methods
3. Each of the constituents of air retain their individual properties
4. Air contains impurities and particulate matter hence contributing to it's homogeneous nature as a mixture
5. Unlike a pure substance with a fixed composition, air's composition can vary based on geographical location, altitude and environmental factors.

Atmospheric oxygen refers to the vital gas present in Earth's atmosphere, primarily in the form of diatomic molecules known as O₂.

This essential element is crucial for sustaining life, as it plays a central role in cellular respiration, enabling organisms to generate energy from nutrients. In the Earth's atmosphere, oxygen constitutes approximately 21% of the air we breathe.

Oxygen is necessary for combustion and corrosion to occur.

Combustion refers to burning. Combustion is a chemical reaction accompanied by production of heat and light. Combustion involves the reaction of combustible materials with atmospheric oxygen leading to production of oxides and other products.

A flame is a region where gases combine chemically, with the production of heat and light. Flames are heterogeneous in nature as they are composed of different zones. A flame may be luminous or non-luminous. The luminosity of a flame increases with a corresponding increase in temperature. The presence of solid particles contribute to the luminosity of a flame. Flames include hydrogen, candle and bunsen flames.

Hydrogen gas burns with a pale blue, non-luminous flame. It consists of two zones: the zone for unburnt gases and the zone of complete combustion.

A candle burns with a yellow luminous flame. The flame has four zones.

1. The zone for unburnt gases around the wick
2. The bright yellow luminous zone (zone of incomplete combustion of the candle)
3. The luminous zone on the outside where there is sufficient amount of oxygen for combustion
4. The blue zone on the base of the flame (region for complete combustion).

A bunsen flame is a much cleaner, hotter and compact flame as compared to the luminous candle flame. It comprises of three distinct zones:

1. The zone of unburnt gases
2. The luminous zone
3. The non-luminous zone

Corrosion of metals is a natural oxidation process of combining metals with atmospheric oxygen and water. The chemical process is sped up by the presence of Carbon (IV) oxide and gaseous pollutants like sulphur (IV) oxide in the air.

The rusting of iron is an example of corrosion of metals. The rusting of iron reduces the quality of iron. Rusting of iron involves the oxidation of iron in the presence of moisture to form reddish brown hydrated iron (III) oxide. $$ 4Fe + 3O_2 + 2xH_2O \rightarrow 2Fe_2O_3.xH_2O $$

The following are ways to prevent rusting of iron

1. Painting: Iron parts in machinery, vehicles and buildings are painted to prevent them from rusting.
2. Greasing and oiling: Rusting can be prevented by periodically coating the exposed iron parts of vehicles and machinery with grease and oil.
3. Galvanizing: This involves coating iron with zinc. The zinc serves as a protective layer over the iron parts thereby preventing corrosion of iron.
4. Electroplating: Iron can be prevented from rusting by coating it with other protective metals like copper, tin, chromium, gold and silver
5. By enamelling: Iron used in making cooking utensils is often coated with enamel. Enamel is a glass-like substance that is put on metal for protection or decoration.

Rusting and burning are both oxidation processes. The common similarities of both processes are as follows:

1. Both processes require oxygen.
2. Both processes give out heat. Combustion produces large amount of heat in a short time while rusting produces small amount of heat over a longer period.
3. Rusting is slow chemical process while burning is an active and rapid process.