Take three test tubes A, B and C and place one clean iron nail in each of them. In the test tube A pour some water and close its mouth with the help of a cork. In the test tube B pour boiled water which does not contain dissolved air. Also pour some oil in test tube B to form a layer over boiled water. The layer of oil will prevent the entry of air in water. In the test tube C put some anhydrous calcium chloride and cork it. Anhydrous calcium chloride is drying agent. So, it is added into test tube C to absorb all the moisture present in the air of test tube. Keep these test tubes undisturbed for some days.

After some days, you will observe that the nail in test tube B does not rust because the nail was exposed only to water and no air was present in it. In the same way the nail present in test tube C does not rust because in this tube air is dry and water is not present. But the nail present in test tube A rusts because in test tube A the nail was exposed to both air and water.

This shows that for rusting of iron both air and water are necessary.

Investigation the conditions under which iron nail rusts

Test your understanding and answer these questions:

1. Give an experiment to prove that both air and water are necessary for rusting of iron.

Air is a mixture of various types of gases and water vapours. The gases and water present in air react gradually with the surfaces of metals and form their oxides, sulphides, carbonates or sulphates. The formation of these compounds eats up metals slowly. This phenomenon is called corrosion of metals. It may be defined as the eating up of a metal by the action of air and moisture on its surface. Usually the process of corrosion takes place in reactive metals. Almost all the reactive metals corrode when they are kept exposed to damp air. For example, when iron is kept exposed in moist air for a long period of time, a brown coloured flaky coating called rust is formed over its surface. Chemically rust is hydrated iron oxide. The chemical formula of rust is Fe2O3. nH2O (where n is the number of water molecules which is variable). It should be noted that the corrosion of iron is called rusting.

In the same way, when copper is kept exposed in moist air for a long time, a green coloured layer deposits over its surface due to reaction of copper with carbon dioxide and water present in moist air. Due to formation of this green coloured layer over the surface of copper, things made up of copper lose their shine after some time. Gold and silver being least reactive do not corrode easily in atmosphere.

In this way we see that corrosion of metals is harmful for us as it destroys a large number of metals. But sometimes the corrosion of metal is useful. For example, when aluminium metal is kept exposed in air, the surface of aluminium is covered by a thin layer of aluminium oxide. This layer of aluminium oxide protects the metal underneath from further corrosion, and hence metal is protected.

Condition Necessary for Rusting of Iron

Following two conditions are necessary for rusting of iron.

1. Presence of air (oxygen)
2. Presence of water

Methods to Prevent Rusting of Iron

Rusting of iron can be prevented by cutting off the contact between the metal and air. Some methods which are used to prevent rusting of iron are

1. Rusting of iron can be prevented by applying paints, oils and grease over the surface of iron.
2. Rusting of iron can be prevented by galvanization. Galvanization is the process of depositing a thin layer of zinc metal on iron articles.
3. Rusting of iron can be prevented by coating its surface with other metals like tin, nickel and chromium.
4. Rusting of iron can be prevented by alloying it with chromium and nickel to make stainless steel.

Test your understanding and answer these questions:

1. What is corrosion of metals?
2. What is rusting of iron?
3. What conditions are necessary for rusting of iron?
4. How rusting of iron metal can be prevented?

An alloy is a homogeneous mixture of one metal with one or more other metals or non-metals. An alloy is prepared by mixing metals in the molten state in a fixed proportion by weight. Various properties of metals like malleability, ductility, strength, hardness, resistance to corrosion and appearance etc. are improved by alloying of metals. For example,

Copper  +  Zinc  =  Brass
Iron  +  Carbon  =  Steel
Copper  +  Tin  =  Bronze

We shall now discuss various alloys formed by different metals.

1. Alloys of Aluminium

From aluminium following alloys are prepared:

Duralumin

It is also known as duralium. It consists of aluminium, copper, magnesium, and manganese. It is light, strong and resistant to corrosion.

Uses of Duralumin

1. Duralumin is used for making aircraft bodies and parts, space satellites and pressure cookers.
2. Duralumin is resistant to water corrosion, so it is used for making bodies of ships.

Magnelium

This alloy of aluminium consists of aluminium and magnesium. It is very light and hard.

Use of Magnelium: It is used to make light instruments and balance beams.

Alnico

It is made up of aluminium, nickel, cobalt and iron.

Use of Alnico: Alnico is used for making magnets.

2. Alloys of Iron

Iron is used to prepare two alloys called steel and stainless steel.

Steel

Steel is a very important alloy of iron which contains 0.1 % to 1.5% carbon. The main properties of steel are that it is hard, tough and strong.

Uses of Steel
1. It is used in construction of buildings and bridges.

2. It is also used to make nails, screws, railway lines and girders.

Stainless Steel

Stainless steel is also an alloy of iron which contains about 18% of chromium and nickel. Stainless steel is very hard and resistant to corrosion.

Uses of Stainless Steel

1. Stainless steel is used for making knives, tools, cooking utensils and ornamental pieces because it is corrosion resistant.
2. It is also used for making surgical instruments such as scissors and various equipments for industrial processes.

3. Alloys of Copper

The important alloys of copper are:

Brass

Brass is prepared by mixing zinc in copper. The main properties of brass are that it is malleable, strong, resistant to corrosion and can be easily cast in different shapes and sizes.

Uses of Brass

1. Brass is used mainly for making cooking utensils, screws, nuts, bolts and scientific instruments.
2. Brass is also used for making flower vases and fancy lamps.

Bronze
This alloy is prepared by mixing tin in copper. It is very strong and resistant to corrosion.

Uses of Bronze: Bronze is used for making statues, medals, cooking utensils and coins.

German Silver

This alloy of copper is prepared by mixing zinc and nickel in copper.

Uses of German Silver: It is used for making utensils, silver ware and for electroplating.

Alloy of Mercury

An alloy of mercury metal with one or more other metal is called as an amalgam. For example sodium amalgam is prepared by mixing sodium metal in mercury.

Alloy of Lead

The most important alloy of lead is solder. It is prepared by mixing tin in lead.

Uses of Solder: It is used for joining metals and wires in electrical work.

Alloy of Gold

Pure gold is very soft due to which it is unsuitable to make jewelry. Purity of gold is measured in carats. Pure gold is of 24 carats. In order to make gold hard and fit for making jewelry copper or silver are mixed in it.

Test your understanding and answer these questions:

1. What are alloys?

Pure Iron is a grey-white metal. It appears to be brown due to rusting. Iron is magnetic, malleable and ductile. It is a good conductor of heat and electricity.

Occurrence of Iron

Iron is the second most abundant metal in earth’s crust. It is not found in a free state in the earth because it is quite reactive. So, it is found in the form of its oxides, carbonates and sulphides. The main ores of Iron are:

1. Haematite      Fe₂O₃
2. Magnetite      Fe₃O₄
3. Siderite         FeCO₃
4. Iron Pyrite     FeS₂

Extraction of Iron from Haematite

Iron is extracted from its chief ore haematite by reducing haematite in a big furnace called a blast furnace. The steps involved in the extraction of iron are:

1. Concentration of Haematite

Haematite is concentrated by the process of hydraulic washing. In this method, haematite is crushed to small pieces and then washed in a stream of water to remove impurities (sand and clay) from it.

2. Calcination

The washed haematite is then heated strongly in the absence of air to expel water from it.

3. Reduction

The washed and dried haematite is now mixed with coke and limestone. The mixture of haematite, coke and limestone is called charge. The charge is then fed into the blast furnace from the top. A blast of the hot air which is at about 800°C temperature is forced in the furnace through tubes present at the bottom. As the hot air rises upward it comes into contact of coke and the following two reactions take place in a blast furnace.

 

1) Formation of Carbon Monoxide

As the hot air comes in contact with coke carbon dioxide is formed. In this reaction, a large amount of heat is produced which raises the temperature of the furnace further to 1500°C. This carbon dioxide rises upward and reacts with more coke to form carbon monoxide. Carbon monoxide is the main reducing agent in the extraction of iron by blast furnace.

C  +  O₂  →  CO₂  +  Heat
Coke  +  Oxygen  →  Carbon dioxide + Heat

CO₂  +  C  →  2CO
Carbon dioxide  +  Coke  →  Carbon monoxide

2) Reduction of Haematite to Iron

The carbon monoxide reduces haematite into iron metal in the upper part of the furnace.

Fe₂O₃  +  CO  →  2Fe  +  3CO₂
Haematite  +  Carbon monoxide  →  Iron Metal  +  Carbon dioxide

The iron so produced is in liquid form and collects at the bottom of blast furnace.

Function of Limestone in Extraction of Iron

The haematite used in the blast furnace always contains sand as impurties. The sand has a high melting point. So, it does not change to liquid in the furnace. Thus, in order to remove sand from iron limestone is added in the blast furnace.

In the blast furnace limestone decomposes to form CaO and CO₂. CaO produced from the lime stone acts as flux. It reacts with sand to form molten calcium silicate referred to as slag.

CaCO₃  +  CaO  →  CO₂
Limestone  +  Calcium oxide  →  Calcium oxide

CaO  +  SiO₂  →  CaSiO₃
Calcium oxide (flux)  +  Sand  →  Calcium silicate (slag)

The molten calcium silicate slag is lighter than iron. So, it floats over molten iron and removed. The iron obtained from the blast furnace is called cast iron or pig iron.

Test your understanding and answer these questions:

1. Name ores of iron metal?
2. Explain the process of extraction of iron metal from haematite ore.
3. What is pig iron?
4. What is flux and slag?

Aluminium is the most abundant metal in the earth’s crust. It is a reactive metal so it occurs in combined state as its oxides and fluorides. The important ores of aluminium are:

1. Bauxite Al₂O₃.2H₂O (aluminium oxide dihydrate)
2. Cryolite Na₃AlF₆ (sodium aluminium fluoride)

The most important ore of aluminium is bauxite. In India bauxite is found in Mumbai, Jammu & Kashmir, Jabalpur, Kolhapur, Mirzapur and Ranchi.

Extraction of Aluminium Metal from Bauxite Ore

1. Purification of Bauxite Ore

Bauxite contains impurities such as sand and iron oxide. These impurities are removed from bauxite by Baeyer’s process. The pure bauxite obtained in Baeyer’s process is called alumina.

2. Electrolysis of Aluminium Oxide

Aluminium metal is extracted by the electrolytic reduction of molten alumina (aluminium oxide) in an iron tank. The melting point of alumina is very high. So, to reduce the melting point of alumina cryolite and fluorspar are added in it.

The mixture of alumina, cryolite and fluorspar is then put into a specially made electrolytic cell. The electrolytic cell is made up of iron which is lined with a layer of carbon from inside. This layer of carbon acts as cathode. A large number of carbon rods are also dipped in the mixture of alumina, cryolite and fluorspar. These rods act as an anode. When an electric current is passed through electrolytic cell, the mixture melts and molten aluminium metal is produced at the cathode and oxygen is liberated at the anode. Finally, the molten aluminium metal collects at the bottom of the iron tank from where it is collected and cooled to make it solid.

Al₂O₃ →  2Al⁺³ +  3O^-2
(Aluminium oxide   →  Aluminium ion  +  Oxide ion)

At Cathode:-

Al⁺³ →^(+3e)  Al
Aluminium ion  →  Aluminium Metal

At Anode:-

O^-2 →^(-2e)  O

O  +  O  →  O₂

During this process, all the carbon anodes are gradually consumed. This is due to the reason that the oxygen gas liberated at anode reacts with carbon anodes to form CO₂ gas. So, it is necessary to replace carbon anodes from time to time.

C(s)  +  O₂(g)  →  CO₂(g)
Carbon  +  Oxygen  →  Carbon dioxide

3. Refining of Aluminium

The aluminium metal produced by the electrolysis of molten aluminium oxide contains some impurities. These impurities can be removed by the electrolytic refining method.

Uses of Aluminium

1. Aluminium is used for making cooking utensils, aluminium foils, television aerials, ships, aeroplanes and space rockets.
2. It is used in aluminotherapy for joining the broken pieces of heavy iron objects like girders, rail or heavy machinery.
3. It is used for making alloys such as duralumin and magnelium.
4. It is used as a reducing agent in metallurgy.
5. It is used in electric transmission lines.

Test your understanding and answer these questions:

1. Name two ores of aluminium metal?
2. Explain the process of extraction of aluminium metal from bauxite ore.
3. What are uses of aluminium metal?

There are two common methods which are used to get metals of very high purity. These are:

1. Zone Refining

By this method, those metals are purified which require high purity such as germanium, silicon and gallium. In this method, impure metal is taken in the form of a rod. A circular heater is fitted around it. The function of the heater is to molt the impurities present in the germanium rod. For this purpose, the heater is slowly moved along the length of the rod to the right side. As the heater moves to the right side on the metal rod, the molten impurities also move to the right side of the rod. Ultimately the impurities reach the extreme right end of metal rod and collect there. This end of rod containing impurities is finally cut off and discarded.

2. Van Arkel Method

This method is used for refining of impure titanium metal. In this method, the impure titanium metal is heated to a temperature of 250°C with iodine. At this high temperature, titanium reacts with iodine to form a volatile compound titanium tetraiodide. Impurities do not react with iodine so these are left behind.

Ti  +  2I₂    TiI₄
(Titanium  +  Iodine  →  Titanium tetraiodide)

The vapours of titanium tetraiodide are then passed over a hot filament of tungsten having a temperature of about 1400°C. At this high temperature, titanium tetraiodide decomposes to form pure titanium metal and iodine is set free.

TiI₄    Ti  +  2I₂
(Titanium tetraiodide  →  Titanium  +  Iodine)

In this way, pure titanium metal is formed which deposits over the tungsten filament and can be removed easily.

Test your understanding and answer these questions:

1. What is zone refining process?
2. What is van arkel method?

The metal obtained by the methods in previous article is usually impure. So, it is to be purified. The method used for refining of metal depends on the nature of metal and impurities present in it. Some common methods which are used for purification of impure metals are:

1. Distillation Method

This method is useful for purification of those volatile metals which have low boiling points such as zinc and mercury. In this method, the impure metal is heated to its boiling point in a vessel. The vapours of metal thus formed are collected and cooled in a separate vessel to get pure metal. The impurities being non-volatile remain behind.

2. Liquation Method

By this method, those metals can be purified which have low melting point. In this method, the block of Impure metal is placed on the top side of sloping floor of a furnace and heated gently. Due to high temperature, the fusible metal melts and flows down to the bottom of the sloping floor while the non-fusible impurities remain behind on the floor. Finally, the pure metal is collected from the bottom of the sloping floor.

3. Electrolytic Refining

Many metals like Cu, Zn, Pb, Cr, Ni, Ag, and Au are refined electrolytically for refining of an impure metal by electrolysis.

We shall understand electrolytic refining of metals by taking the example of refining of copper. In case of copper, a thick block of impure copper is made anode and a thin block of pure metal is made cathode and copper sulphate solution is used as an electrolyte. On passing electric current, pure copper metal from the electrolyte solution deposits on the cathode. At the same time, an equal amount of impure copper dissolves from anode into the electrolyte solution. The soluble impurities settle down in the solution below the anode and are called as anode mud.

Test your understanding and answer these questions:

1. Explain electrolytic refining of copper metal.

The process of converting metal oxides into metals is called reduction. For reduction, different types of reducing agents are used depending on the reactivity of ores. The commonly used methods for reduction are :

1. Reduction by Heat (pyrometallurgy)

The oxides of metals which are present at the bottom of reactivity series can be reduced to metals by the action of heat alone e.g. mercury oxide can be reduced to mercury metal by heating it to a temperature of about 300°C.

2HgO             2Hg      +      O₂
Mercury oxide  →  Mercury metal + Oxygen

2. Reduction by Coke (smelting)

The Oxides of Metals like Zn, Fe, Cu, Ni, Sn and Pb are usually reduced by using carbon as reducing agent. In this process, coke is mixed with roasted ore and heated to a high temperature in a furnace. Coke reduces the metal oxides into free metal. For example,

ZnO  +  C  →  Zn  +  CO
(Zinc oxide  +  Carbon  →  Zinc  +  Carbon monoxide)

PbO  +  C  →  Pb  +  CO
(Lead oxide  +  Carbon  →  Lead  +  Carbon monoxide)

3. Reduction by Aluminium (aluminotherapy)

Oxides of manganese and chromium metals are reduced to metals with the help of Aluminium. The process of reduction of a metal oxide to the metal with the help of aluminium is called aluminotherapy.

3MnO₂  +  4Al  →  3Mn  +  2Al₂O₃
(Manganese dioxide  →  Manganese metal  +  Aluminium oxide)

4. Electrolytic Reduction

The Oxides of metals which are quite high in reactivity series can be reduced to metals by electrolytic reduction. For example, sodium and magnesium metals are obtained by electrolytic reduction of their chloride solutions in a molten state.

2NaCl  →  2Na  +  Cl₂
(Sodium chloride  →  Sodium metal  +  Chlorine gas)

MgCl₂  →  Mg  +  Cl₂
(Magnesium chloride  →  Magnesium metal  +  Chlorine gas)

During this process, chlorine gas is liberated at anode while metals (sodium or magnesium) deposit at cathode.

Test your understanding and answer these questions:

1. What is pyrometallurgy?
2. What is smelting process?
3. Define aluminotherapy.
4. What is electrolytic reduction? explain.

After the concentration of ores, the sulphide or carbonate ores of some metals are converted into metal oxides because it is easier to obtain metals from their metal oxides as compared to metal sulphides or metal carbonates. The sulphide ores of metals can be converted into their oxides by roasting while the carbonate ores of metals can be converted into their oxides by calcinations.

1. Roasting

It may be defined as the process of strongly heating a sulphide ore in the presence of air to convert it into metal oxide. For example,

2ZnS  +  3O₂          2ZnO  +  2SO₂
(Zinc sulphide + Oxygen → Zinc Oxide + Sulphur dioxide)

2PbS      +      3O₂            2PbO      +      2SO₂
Lead sulphide + Oxygen → Lead Oxide + Sulphur dioxide)

2. Calcinations

It may be defined as the process of strongly heating a carbonate ore in the absence of air to convert it into metal oxide. For example,

CaCO₃            CaO      +      CO₂
(Calcium carbonate  →  Calcium oxide + Carbon dioxide)

Test your understanding and answer these questions:

1. What do you understand by calcination and roasting of ores?

Before extracting the metal from an ore, it is necessary to remove the impurities present in it. By removing these impurities we get a concentrated ore which contains a high percentage of metal in it. The process of removal of gangue particles from ore to increase the percentage of metal in ore is called enrichment of ore. The processes used for removing the impurities from ores depend on the difference between physical or chemical properties of the ore and of the impurities.

Methods of enrichment of ore:

Now, we shall discuss the different processes which are used for the enrichment of different types of ores.

1. Hydraulic Washing

This method is used for the enrichment of those ores which are heavier than gangue particles present in them. In this method, a stream of water is passed through crushed and finely powdered ore. The Lighter gangue particles are washed away with water while the heavier ore particles are left behind. Oxide ores of tin and lead are concentrated by this method.

2. Froth Floatation Process

This method is used for concentration of sulphide ores of copper, lead and zinc. In this method, powdered ore is put in a tank full of water. And then, some Pine oil is added to it. In the tank, the particles of sulphide ore are wetted by pine oil whereas the gangue particles are wetted by water. Then the air is passed through this mixture. This results in the agitation of water in the tank, which causes the sulphide ore particles to stick with the oil and rise to the surface in the form of froth. The gangue particles being heavier remain behind at the bottom of the water tank. The froth is separated and concentrated sulphide ore is obtained from it.

3. Magnetic Separation

This method is used for the concentration of magnetic ores of iron (magnetite and chromite) and manganese (pyrolusite) by removing non-magnetic impurities present in them. This process involves the use of a magnetic separator.

A magnetic separator consists of a leather belt which moves over two rollers. Out of two rollers, one roller has a magnet in it. In this method, the finely powdered magnetic ore is dropped over the moving belt at one end. When the powdered ore falls down from the moving belt at the other end having a magnetic roller, the particles of ore are attracted by the magnet and form a separate heap from the non- magnetic impurities.

4. Chemical Separation

This method is based on the differences in some chemical properties of the gangue and the ore. For example, the impure ore of metal aluminium (bauxite or aluminium oxide) is concentrated by Baeyer’s process.

Baeyer’s Process

In Baeyer’s process, the finely powdered bauxite ore is treated with hot sodium hydroxide solution to form a water soluble compound called sodium aluminate.

Al₂O₃  +  2NaOH   →   2NaAlO₂   +   H₂O
(Bauxite  +  Sodium hydroxide  →  Sodium aluminate  +  Water)

The gangue present in bauxite does not react in sodium hydroxide sol, so the gangue can be separated by the process of filtration. After filtration, the filtrate containing a solution of sodium aluminate is acidified with HCl to form precipitates of aluminium hydroxide.

NaAlO₂  +  HCl   +  H₂O   →   Al(OH)₃  +  NaCl
(Sodium aluminate + Hydrochloric acid + Water →  Aluminium hydroxide + Sodium chloride)

The precipitates of aluminium hydroxide are then filtered, washed, dried and heated to get pure aluminium oxide.

2Al(OH)₃  →  Al₂O₃  +  3H₂O
(Aluminium Hydroxide  →  Aluminium Oxide + Water)

It should be noted that pure aluminium oxide is also known as alumina.

Test your understanding and answer these questions:

1. What do you understand by enrichment of ore?