Oxygen, O2, is a colourless odourless gaseous main group element
which belong to
Group VIb of the
Atmospheric oxygen is of vital importance for all aerobic organisms.
For industrial purposes, oxygen is obtained by fractional distillation
of liquid air. It is used in metallurgical processes,
in high-temperature flames and in breathing apparatus.
- Atomic Number : 8
- Atomic Mass : 15.9994
- Melting Point : -214 degC
- Boiling Point : -183 degC
- Density : 1.429
The discovery of Oxygen was credited to Priestley in 1774 AD.
However in a paper looking into Alchemy,
by Richard Brzezinski, an expert in the history of science and Zbigniew Szydlo, a chemistry
lecturer, published in the authoritative magazine History Today credit the discovery of
Oxygen to a Polish alchemist called Michael Sendivogius who found that heated saltpeter
produced "the elixir of life" and who, in 1604, described his experiments in a book regarded
as so authoritative that it found its way into every major scientific library in Europe.
They say that Priestley, would surely have had access to it. Cornelis Drebbel a Dutch
inventor employed by the King of England James 1 in 1621 used Sendivogius work which
was about 150 years before Priestley was credited with the discovery of Oxygen. Drebbel
built a submarine which was manned by 12 oarsmen, made of wood and waterproofed by
a coat of greases leather. It successfully traveled along the River Thames from Westminster
to Greenwich, at a dept of 15 ft. The trip, and the method used to keep the oarsmen alive,
was subsequently verified by Robert Boyle.
Oxygen occurs in the free state as a gas, to the extent of
21 per cent by volume or 23 per cent by weight in the atmosphere.
Combined Oxygen also occurs
Oxygen occurs to a larger extent in the earth's crust than
any other element.
- in water,
- in vegetable and animal tissues,
- in nearly all rocks and
- in many minerals.
Because oxygen is a component of air, it has been studies extensively
over the centuries and there is a large number of different methods
for its preparation.
The most convenient method for preparing oxygen in the laboratory
involves either the catalytic decomposition of solid potassium chlorate
or the catalytic decomposition of hydrogen peroxide.
Preparation of oxygen Using potassium chlorate
Potassium chlorate decomposes at a low temperature if previously
mixed with manganese dioxide which is a catalyst
for the decomposition.
Only the potassium chlorate is decomposed, and no perchlorate is formed :
2 KClO3 ==> 2 KCl + 3 O2
Preparation of oxygen using hydrogen peroxide
The decomposition of hydrogen peroxide using manganese dioxide
as a catalyst also results in the production of oxygen gas.
2 H2O2 ==> 2 H2O + O2
Preparation of oxygen by electrolysis of water
The electrolysis of acidified
is carried out in a Hofmann
Oxygen is evolved at the positive electrode in the electrolysis.
2 H2O ==> 2 H2 + O2
A solution of barium hydroxide with nickel electrodes may also be used.
However, on prolonged electrolysis an explosive mixture of oxygen and
hydrogen may be evolved at the positive electrode.
Preparation of oxygen by the chemical decomposition of water
Oxygen is obtained from water by passing a mixture of steam and chlorine
through a strongly heated silica tube containing pieces of broken
2 H2O + 2 Cl2 ==> 4 HCl + O2
The hydrogen chloride is removed by a wash-bottle containing
sodium hydroxide solution and the Oxygen collected over water.
Preparation of oxygen By decomposition of oxides
Oxygen may be obtained by heating some metallic oxides.
Preparation of oxygen by the decomposition of salts
Some salts containing oxygen decompose and release oxygen gas on heating.
- Potassium nitrate melts on heating and at a slightly high
temperature decomposes, giving off bubbles of oxygen and forming
potassium nitrite which solidifies on cooling.
2 KNO3 ==> 2 KNO2 + O2
- Potassium chlorate crystals melt when heated in a hard
glass tube at 360 degC and then decompose to form potassium
chloride and releasing oxygen.
2 KClO3 ==> 2 KCl + 3 O2
- Potassium permanganate which is a purple crystalline solid,
decomposed without fusing on heating to 240 degC, forming a
black powder consisting of a mixture of potassium manganate and
manganese dioxide and releasing oxygen.
2 KMnO4 ==> K2MnO4 + MnO2 + O2
- Potassium permanganate explodes violently when heated with
concentrated sulphuric acid. However, when a solution of
hydrogen peroxide is mixed with a solution of the permanganate and
diluted sulphuric acid added, the two compounds decompose together,
forming a nearly colourless solution, and oxygen is evolved.
2 KMnO4 + 3 H2SO4 + 5 H2O2 ==> K2SO4 + 2MnSO4 + 8H2O +5O2
- Chromic trioxide which is a red crystalline solid, melts on
heating at about 420 degC, leaving a green residue of chromic oxide
and evolves oxygen.
4 CrO3 ==> 2 Cr2O3 + 3 O2
- Potassium dichromate which is a bright-red crystalline solid,
melts on heating and when strongly heated releases oxygen
leaving a mixture of yellow potassium chromate which is
soluble in water, and green chromic oxide, which is insoluble in water.
4 K2Cr2O7 ==> 4 K2CrO4 + 2 Cr2O3 + 3 O2
- Chromium trioxide and potassium dichromate when heated with
concentrated sulphuric acid forms chromic sulphate and releases oxygen.
4 CrO3 + 6 H2SO4 ==> 2 Cr2(SO4)3 + 6 H2O +3 O2
2 K2Cr2O7 + 10H2SO4 ==> 4 KHSO4 + 2 Cr2(SO4)3 + 8 H2O +3 O2
Preparation of oxygen from air
Oxygen may be obtained from the atmosphere in a chemical process, by
heating mercury in a confined volume of air, when the oxygen reacts
with the mercury to form mercuric oxide. The mercuric oxide so
formed is then heated strongly, when it decomposes and pure oxygen
In a similar process, if yellow lead monoxide is carefully heated
in an iron dish and freely exposed to air, it takes up oxygen from
the air and forms red lead.
6 PbO + O2 ==> 2 Pb3O4
On heating strongly, the red lead decomposes into lead monoxide and
Oxygen gas which is evolved.
2 Pb3O4 ==> 6 PbO + O2
Various methods have been used for the large scale production of oxygen,
but at present the two mostly used are the electrolysis of an aqueous
solution of dilute sulphuric acid, and the fractional distillation of
Manufacture from liquefied air
Oxygen may be obtained from the atmosphere by the liquefaction and
fractional distillation of air. Liquid air is a mixture of
liquid nitrogen, boiling point -196 degC, and liquid oxygen, boiling
point -183 degC. The nitrogen is more volatile (i.e. it has a lower
boiling point) and boils off first during evaporation. Because
some oxygen evaporates with the nitrogen, separation of the two gases
is brought about by fractionation (i.e. by letting the evolved gas
mixture bubble through liquid air rich in oxygen in a tall
rectifying column). The oxygen in the gas mixture condenses and
almost pure nitrogen gas leaves the top of the column, leaving
almost pure liquid oxygen which is then evaporated to give oxygen gas.
The oxygen gas is distributed as a compressed gas in high pressure
- a colourless gas, without smell or taste,
- is slightly heavier than air,
- is sparingly soluble in water,
- is difficult to liquefy, boiling point -183 degC, and the
liquid is pale blue in colour and is appreciably magnetic.
At still lower temperatures, light-blue solid oxygen is obtained,
which has a melting point of -218.4 degC.
Oxygen is essential for life and it takes part in processes of
combustion, its biological functions in respiration make it important.
Oxygen is sparingly soluble in water, but the small quantity of
dissolved oxygen in is essential to the life of fish.
Oxygen gas is used with hydrogen or coal gas in blowpipes and with
acetylene in the oxy-acetylene torch for welding and cutting metals.
Oxygen gas is also used in a number of industrial processes.
Medicinally, oxygen gas is used in the treatment of pneumonia
and gas poisoning, and it is used as an anesthetic when mixed
with nitrous oxide, ether vapour, etc..
Carbon Dioxide is often mixed with the oxygen as this stimulates
breathing, and this mixture is also used in cases of poisoning
and collapse for restoring respiration.
Liquid oxygen mixed with powdered charcoal has been used as an explosive.
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