Oxygen was first discovered by Scheele and Priestly in 1774. Oxygen is the first element in group (VI) in the periodic table. It is a diatomic molecule and the most abundant element on earth.

Free oxygen consists of about 21% of atmospheric air. In combined state, oxygen is present in trioxocarbonates, trioxonitrates and oxides of metals and nonmetals.

Water, one of the most abundant compounds on earth is made up of 88% oxygen by mass.

Oxygen forms ionic compound by accepting two electrons from metals to form negative ion, \( O^{2-} \) (forms ionic compounds). $$ Na^+ + O^{2-} \rightarrow Na_2O $$ It also forms single covalent bond with two atoms of hydrogen to yield water $$ H — O — H $$ In oxygen molecule, a double covalent bond joins the two oxygen atoms $$ O \rightleftharpoons O $$

1. Thermal decomposition of potassium trioxochlorate (V) : The thermal decomposition of potassium trioxochlorate (V) yields oxygen gas. Manganese (IV) oxide is used as catalyst to speed up the rate of the reaction because the temperature is lowered.

   The gas is collected over water because it is denser than air and dried using dehydrating agent like fused calcium chloride or concentrated Sulphuric acid. $$ 2KClO_3 \rightarrow 2KCl + 3O_2 $$

2. Decomposition of hydrogen peroxide: This requires no heat. On adding manganese (IV) oxide to hydrogen peroxide, it decomposes to liberate oxygen gas. $$ 2H_2O_2 \rightarrow 2H_2O + O_2 $$

The main processes for the industrial preparation of oxygen are:

1. Liquefaction of liquid air
2. Fractional distillation of air

First, Carbon dioxide is removed by passing it through caustic soda. In the liquefaction of air, the process begins by cooling air to temperatures below its critical point, causing its components to condense into a liquid state. Gaseous air becomes liquid at -200°C

Next, the liquid air undergoes fractional distillation to separate its components. As the liquid air is gradually warmed:

1. Nitrogen, with a lower boiling point of approximately -196°C, vaporizes first.

2. The vaporized nitrogen is collected, leaving behind a liquid enriched in oxygen.

3. Oxygen, having a higher boiling point of around -183°C, vaporizes later in the process.

4. The vaporized oxygen is then collected, completing the separation of nitrogen and oxygen based on their distinct boiling points.

The boiling point difference (-196°C for nitrogen and -183°C for oxygen) facilitates this fractional distillation process.