Electrolysis is a process
in which an electric current is passed through a liquid,
causing a chemical reaction to take place. If the liquid
is water, electrolysis "breaks up" the water into
two gases--hydrogen and oxygen. If the liquid is a solution
that contains a metal, electrolysis breaks up the solution
so that the metal is removed. The electrolysis of metallic
solutions is useful in putting metal coatings on objects,
and in refining, or purifying, metals.
How electrolysis works.
To produce electrolysis, two solid electrical conductors,
such as metal or graphite rods, are placed into a liquid.
The rods are called electrodes. Wires connect the electrodes
to the terminals of a battery or to a direct current generator.
The liquid must contain a substance, called an electrolyte,
that enables it to carry the current and complete the electrical
circuit. For example, distilled water cannot be electrolysed
because it does not conduct electricity well. But it can
be electrolysed if a little table salt (sodium chloride),
which is an electrolyte, is added to it. The electrodes,
the liquid, and the container that holds them make up an
The electrode connected to the battery's negative pole
is the cathode. It carries electrons from the battery to
the electrolytic cell. The electrode connected to the battery's
positive pole is the anode. It carries electrons from the
electrolytic cell back to the battery.
As the current flows through the electrolytic cell, chemical
changes take place at the surfaces of the electrodes. At
the cathode, the electrolysed liquid combines with electrons
supplied by the battery. This process is called reduction.
At the anode, the liquid gives electrons to the anode. This
process is called oxidation.
In the electrolysis of water, the water combines with electrons
at the cathode and is reduced to hydrogen gas. At the anode,
water gives up electrons and is oxidized to oxygen gas.
The volume of the hydrogen produced is always twice the
volume of the oxygen produced, because water contains two
atoms of hydrogen for each atom of oxygen.
In the electrolysis of solutions containing ions (charged
atoms) of such metals as copper and silver, the reduction
of the metal at the cathode causes the metal to be deposited,
or to plate out, on the cathode.
Uses of electrolysis.
The reduction of metals from their compounds by the process
Examples : Sodium metal is produced by
the electrolysis of molten sodium chloride. In this process,
chlorine gas is produced at the anode. Both sodium metal
and chlorine gas have many important industrial and chemical
uses. The electrolysis of sodium chloride dissolved in water
forms another important chemical, sodium hydroxide (caustic
Magnesium, aluminium, and certain other metals are produced
commercially by electrolysis. Aluminium metal is
made by electrolysis of alumina dissolved in the molten
The process of purifing metals extracted by checmical processes
by the process of electrolysis. In this anode is made from
the impure metal which has to be purified . The cathode
is made of sheets of pure metal. The electrolyte is of a
compound of the metal being refined. The cathode increases
in mass and anode decreases. The impurities either get dissolved
in the electrolyte or get deposited at the bottom of the
vessel as anode mud.
Copper and other minerals are purified by electrolysis.
A bar of impure copper is made the anode and a bar of pure
copper is made the cathode. During electrolysis, the impure
copper anode is dissolved into copper ions. The pure copper
it contains plates out on the cathode. All impurities from
the anode fall to the bottom of the electrolytic cell and
are removed later.
Important chemicals produced commercially by electrolysis
include manganese dioxide, hydrogen peroxide, chlorates,
and perchlorates. Hydrogen peroxide and perchlorates are
used in rocket fuels. Electrolysis is also used to anodize
metals to make their surfaces more decorative and resistant
Laws of electrolysis.
The English chemist Michael Faraday was one of the first
scientists to investigate electrolysis. After many careful
experiments and calculations, he stated the following three
"laws" governing electrolysis:
1. The ability of an electric current to cause electrolysis
does not depend on the distance between the electrodes.
2. The quantity of a substance that is electrolysed is
proportional to the quantity of the electricity used.
3. The quantity of a substance that is electrolysed is
also proportional to the substance's chemical equivalent.
The chemical equivalent of a metal is its atomic weight
(in grams) divided by its valency.
Faraday found that approximately 96,500 coulombs of electricity
are required to electrolyse one chemical equivalent of any
metal. For example, the atomic weight of copper is 63.54,
and the valence of copper salts is +2. Therefore, the chemical
equivalent of a copper salt solution is 31.77 grams. This
amount of copper will plate out onto the cathode when 96,500
coulombs are passed through the solution.
The number of coulombs that flow in each second is measured
in units called amperes. Voltage is like an electrical pressure
that pushes the coulombs through the circuit. In electrolysis,
voltage is just as important as amperage. A certain minimum
voltage is needed to produce electrolysis in any given substance.
For example, a minimum of 1.23 volts is needed to electrolyse
water to hydrogen and oxygen at 25 °C.