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Dioxygen – Definition, Properties, Preparation, Uses

  • Last Updated : 28 Nov, 2021

Oxygen is a member of the periodic table’s chalcogen group, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements and other compounds. Oxygen is the most abundant element on Earth, and it is the third-most abundant element in the universe after hydrogen and helium. Diatomic oxygen gas now accounts for 20.95% of the Earth’s atmosphere. Oxygen, in the form of oxides, accounts for nearly half of the Earth’s crust.

The chemical element with the symbol O and atomic number 8 is oxygen. 


One of the most common allotropes of elemental oxygen is dioxygen, which has the chemical formula O2. It is commonly referred to as oxygen, but to distinguish it from elemental oxygen, it is also referred to as dioxygen, molecular oxygen, or oxygen gas. 

With the exception of noble gases, oxygen gas reacts with almost all elements. The resulting compound is referred to as oxides. Even though it is not flammable on its own, oxygen gas is critical for combustion. It is also a life-giving gas because mammals require oxygen to survive and it aids in the release of energy.

Physical Properties of Dioxygen

  1. It’s a gas with no odour, colour, or taste.
  2. With a density of 1.429 g/L, it is heavier than air.
  3. It is slightly soluble in water, just enough to support aquatic life.
  4. The melting point of oxygen is approximately 54.36 K, and the boiling point is approximately 90.188 K.
  5. Depending on the temperature and pressure, oxygen can exist in all three states: solid, liquid, and gas.

Chemical Properties of Dioxygen

  • It reacts directly with almost all metals and nonmetals to form oxides of the elements involved.

4Na + O2 → 2Na2O (With Metal)

C + O2 → CO2 (With Non-metal)    

  • It has a paramagnetic nature.
  • Normally, oxygen does not react with acids and bases.
  • Because oxygen is a good oxidant, it aids in combustion.

Fuel + O2 ⟶ CO2 + H2O

  • The formation of rust on iron is caused by the combination of oxygen and moisture.

Fe  + O2 + H2O  ⟶ Fe2O3.nH2O (Hydrated Iron Oxide)

Laboratory Preparation of Dioxygen

In the laboratory, dioxygen can be prepared in a variety of ways.

  • The catalytic decomposition of Sodium Potassium Chlorate with Magnesium dioxide yields dioxygen. This reaction takes place when heated in the presence of MnO2 at 420K.

2KClO3  →  2KCl + 3O2  

  • The thermal breakdown of metal oxides with low electrode potential in the electrochemical series, such as Mercury and Silver oxides, produces dioxygen.

2HgO (s) → 2Hg (l) + O2 (g)

2PbO2 (s) → 2PbO (s) + O2 (g)

  • When oxygen-rich salts, such as nitrates and permanganates, are thermally decomposed, Dioxygen is produced.

2KNO3 →  2KNO2+ O2

2KMnO4 → K2MnO4  + MnO2 + O2

2NaNO3  → 2NaNO2  + O2

  • The decomposition of Hydrogen Peroxide also produces oxygen, and manganese(IV) oxide is used as a catalyst to speed up the decomposition.

2H2O2(aq) → 2H2O(l) + O2(g)

Industrial Production of Oxygen

There are two primary methods for industrially producing dioxygen from the air.

  1. N2 distils as a vapour while O2 remains as a liquid in a fractional distillation of liquified air. In this case, liquid air is a mix of liquid Nitrogen and liquid Oxygen. Because of its lower boiling point, nitrogen is more volatile. It boils first, leaving only pure oxygen behind.
  2. Another method involves passing clean, dry air through one bed of a pair of zeolite molecular sieves, which absorbs the N2 gas and delivers the gas, which is 90-93% oxygen.

Uses of Dioxygen

  • Dioxygen is required for respiration.
  • It is found in oxygen cylinders used in hospitals and mountaineering.
  • In the form of oxy-acetylene, it is used for welding and cutting metals.
  • When oxygen gas reacts with acetylene gas, an oxy-acetylene flame is formed, which is used for cutting and welding metals.
  • It is used in liquid rocket fuel.
  • It is used in the synthesis of nitric acid.
  • It is used in artificial respiration in conjunction with carbon dioxide or methane.
  • Laser cutting makes use of oxygen.
  • In combustion processes, oxygen is used. Materials that do not normally burn in air burn easily in oxygen, so combining oxygen and air improves combustion.
  • Oxygen is used in water treatment processes such as wastewater purification and sewage treatment.

Sample Questions

Question 1: Why is dioxygen a gas but sulphur a solid? 


The O—O bond is weaker than the S—S bond due to greater inter-electronic repulsions in small oxygen atoms; additionally, because oxygen is smaller and more electronegative, it forms p-p multiple bonds. As a result, it exists as O2 molecules held together by a weak van der Waals force. At room temperature, oxygen exists as a gas. Sulphur has a lower proclivity to form p-p multiple bonds. Furthermore, it has a large atomic size, low electronegativity, and forms strong S—S single bonds, which explains why it has higher catenation and exists as S8 molecules with a puckered ring structure. As a result, at room temperature, sulphur is a solid.

Question 2: Does Oxygen react with Alkali Metals?


Oxygen has a high reactivity with alkali metals (Group I elements). Alkali metals should be kept away from oxygen to avoid being oxidised. Metals at the bottom of the group are more reactive than those at the top.

Question 3: What is the use of dioxygen?


Dioxygen is required for respiration. It is found in oxygen cylinders used in hospitals and mountaineering. In the form of oxy-acetylene, it is used for welding and cutting metals.

Question 4: Is dioxygen considered a molecule or a compound?


It is a diatomic molecule made up of two oxygen atoms joined by a covalent bond.

Question 5: Dioxygen is a blue-coloured gas and is very essential for life. True or false?


The oxygen we breathe is in the form of a diatomic molecule known as dioxygen gas. It is a gas that is colourless, odourless, and tasteless. It is required for life’s sustenance because plants and animals use it in the process of respiration, which breaks down food molecules to produce energy. As a result, the given statement is false.

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