Metals of high reactivity, such as sodium, calcium, magnesium, aluminium, etc. are extracted from their ores by electrolytic reduction. These metals cannot be reduced using carbon because carbon is less reactive than them.
Electrolytic Reduction: Electric current is passed through the molten state of metal ores. Metal (being positively charged) is deposited over cathode.
Example: When electric current is passed through molten state or solution of sodium chloride, sodium metal is deposited over cathode.
Na+ + e− ⇨ Na
2Cl− − e− ⇨ Cl2
2NaCl ⇨ 2Na + Cl2
Metals obtained from the process of electrolytic reduction are pure in form.
Metals extracted from various methods contain some impurities, and hence are required to be refined. Most of the metals are refined using electrolytic refining.
Electrolytic Refining: In the process of electrolytic refining a lump of impure metal and a thin strip of pure metal are dipped in the salt solution of metal to be refined. When electric current is passed through the solution, pure metal is deposited over thin strip of pure metal, from lump of impure metal. In this, impure metal is used as anode and pure metal is used as cathode.
Fig Ref: NCERT Textbook Class 10 Science
A lump of impure copper metal and a thin strip of pure copper are dipped in the solution of copper sulphate. Impure lump of metal is connected with the positive pole and thin strip of pure metal is connected with the negative pole. When electric current is passed through the solution, pure metal from anode moves towards cathode and is deposited over it. Impurities; present in metal are settled near the bottom of anode in the solution. Settled impurities in the solution are called anode mud.
Cu − 2e− ⇨ Cu+ +
Cu+ + + 2e− ⇨ Cu
Rust is formed when iron reacts with atmospheric moisture. Many metals react with air to develop a layer of oxide and other compounds. In due to course of time, conversion of metal to its compounds eats away the metal. This process is called corrosion. Corrosion results in loss of metal article. Thus, corrosion results in loss of valuable resources.
Rusting of Iron: Rusting of iron is the most common form of corrosion. When iron articles; like gate, grill, fencing, etc. come in contact with moisture present in air, the upper layer of iron turns into iron oxide. Iron oxide is brown-red in color and is known as rust. This phenomenon is called rusting of iron.
If reaction between metal and atmospheric factors can be prevented, we can prevent rusting. Rusting of iron can be prevented by painting, oiling, greasing, galvanising, chrome plating, anodising or making alloys.
Galvanisation involves coating a thin layer of zinc on iron article. The coating of zinc prevents chemical reaction between iron and atmospheric factors. Thus, it prevents rusting.
Alloying also helps in preventing rusting. Alloy is a homogeneous mixture of a more than one metal or a mixture of a metal and non-metal. Alloy imparts certain desirable properties to metal. To understand this, let us take example of iron. Iron is never used in pure form, because iron is very soft and easily stretches when it is hot. But when iron is mixed with a small amount of carbon (0.05%), it becomes hard and strong. Mixture of iron and carbon is called steel. Mixture of iron and nickel or chromium is called stainless steel. This is an alloy which does not rust.
In this section, you read about extraction of metals of high reactivity. You read about electrolytic refining of copper and purification of metals by this process. You read about rusting and corrosion and economic loss because of corrosion. At the end of the section, you read about various methods of prevention of rusting.
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