Revision Notes for Metallurgy Class 10 Chemistry ICSE

ICSE Revision Notes for Metallurgy Class 10 Chemistry

Chapter Name	Metallurgy
Topics Covered	Properties of Metals Properties of Non-metals  Reaction of Metals with Salt Solutions of other Metals The Reactivity Series Corrosion: Cause and Prevention Concentration of Ores Metallurgy of Aluminium Extraction and Refining of Metals Uses of Metals Uses of Non-Metals Uses of Alloys
Related Study	Selina Chemistry ICSE Solutions for Class 10 Frank ICSE Solutions for Class 10 Chemistry

Physical Properties of Metals and Non-Metals

Do you know how many elements are there in our periodic table? 

There are 118 elements in the modern periodic table. These elements can be broadly  classified as metals and non-metals depending on their properties. 

Elements that lose electrons to form compounds are called metals whereas elements  that gain electrons to form compounds are called non-metals. Elements such as Si,  Ge, As, Sb and Te show the characteristic properties of both metals and non-metals.  They are called semi-metals or metalloids. Here, we will discuss metals and non metals along with their physical properties in detail. 

Metals 

These elements are electropositive and contain less than or equal to three electrons in  their valence shell. Metals such as aluminium, copper, and iron are widely used around  us. Metals are used for the construction of bridges, automobiles, airplanes, ships, trains,  etc.

Physical properties of metals: 

1. Metallic Lustre: The surface of most metals is shiny. The lustre associated with  metals is known as metallic lustre. For example, iron, copper, gold, and silver are very  shiny. Metals such as gold and silver are very lustrous. Therefore, they are used for  making jewellery. 

Silver is used for making mirrors because of its excellent shine and reflective nature.

Do you know that metals like gold, silver, platinum, paladium and rhodium are known  as noble metals. They occur in the elemental state in nature. 

Some metals do not look very lustrous. This is because they either lose their lustre or  their lustre gets reduced when exposed to air for a long time. This happens due to the  formation of a layer of oxide, carbonate, and sulphide on their surface. If a metal surface  is rubbed with sand paper, then this layer gets removed and the shiny surface of the  metal can be seen.  

The layer formed in some cases is stable and sticks on the surface of the metal, but in  other cases, it is unstable and falls off (as in the case of rusting of iron).

2. Hardness: Metals are generally hard in nature. However, this hardness varies from  metal to metal. Most metals such as iron, aluminium, etc. are very hard and cannot be  cut with a knife whereas some metals such as sodium and potassium are very soft and  can be cut using a knife. 

3. Malleability: Metals are malleable. Most metals such as iron, copper, silver, and gold  can be hammered without breaking to form thin sheets. Aluminium, and silver are highly  malleable metals and are often used for making foils, which are extensively used in the  decoration of sweets, packing of food items, etc. 

4. Ductility: Most metals are ductile, which means that they can be drawn into thin  wires without breaking. For example, iron, copper, silver, and gold can be drawn into  thin wires without breaking. For this reason, copper and aluminium are extensively used  for making electrical wires. 

Gold and silver are the most malleable and ductile metals. Hence, they are  extensively used in jewellery.

5. Conduction of heat: Metals are generally good conductors of heat. This means that  if one end of a metal rod is heated for some time, then the entire rod becomes hot. For  example, aluminium, copper, and silver are good conductors of heat. Hence, copper  and aluminium are generally used for making vessels. The following activity can be  performed to explain that metals can conduct heat. 

6. Conduction of electricity: Metals are good conductors of electricity i.e., they allow an electric current to pass through them easily. Silver, copper, and aluminium are the  best conductors of electricity. For this reason, most electric wires are made of copper  and aluminium. However, using silver for making electric wires is not cost effective. The following activity can be performed to explain that metals can conduct electricity.

7. Melting and boiling points: Melting and boiling points of metals are usually high. 

8. Physical state: All metals exist as solids at room temperature except mercury, which  exists as a liquid. 

9. Sonority: Metals such as iron and copper produce a sound on being struck. Hence,  metals are said to be sonorous. 

Non-metals 

Many elements in the periodic table do not behave like metals. These elements are  known as non-metals. These elements gain electrons to form compounds. These are  electronegative and contain more than three electrons in their valence shell. Carbon,  sulphur, iodine, oxygen, etc. are some examples of non-metals. Non-metals exist in all  three physical states i.e., as solids, liquids, and gases. Bromine is the only non-metal,  which exists as a liquid. 

Physical properties of non-metals: 

1. Lustre: Non-metals do not have a shiny surface. However, iodine is an exception,  which has a very shiny surface. 

2. Hardness: Non-metals generally exist as solids, liquids, or gases. Non-metals that  exist in a solid state are very soft. For example, sulphur, which exists in solid state, is  quite soft. Similarly, carbon, in the form of graphite, is quite soft. However, diamond,  another allotrope of carbon, is very hard. It is in fact the hardest known natural  substance. 

3. Malleability and ductility: Non-metals that exist in solid states are not very strong.  They are brittle and break when pressure is applied on them. Therefore, non-metals are  neither malleable nor ductile.

4. Conduction of heat and electricity: Non-metals are poor conductors of heat and  electricity. Examples include sulphur and phosphorus. However, there is an exception.  Graphite, an allotrope of carbon, is a good conductor of electricity. 

5. Physical state: Non-metals exist in all three physical states at room temperature.  Non-metals such as carbon, sulphur, and phosphorus exist in solid states while oxygen,  chlorine, and nitrogen exist in gaseous states. Bromine is the only non-metal that exists  in a liquid state. 

6. Melting and boiling points: Melting and boiling points of non-metals are quite low.  For example, the melting point of phosphorus is 44.2°C. However, diamond, an  allotrope of carbon, is the only non-metallic substance that has a very high melting and  boiling point. The melting point of diamond is more than 3500°C. 

7. Sonority: Non-metals are not sonorous. 

The given table summarizes the properties of metals and non-metals. 

Metals	Non-metals
Metals are very hard and strong.	Solid non-metals are soft and can be easily broken.
Metals have a shiny lustre.	Non-metals are not shiny and have a dull appearance.
Metals are sonorous.	Non-metals are not sonorous.
Metals are malleable and ductile	Non-metals are neither malleable nor ductile.

Reaction of Metals with Solution of Salts of other Metals

Categorisation of Metals 

Based on the positioning of metals in the periodic table, their characteristics are given below:

	Alkali Metals	Alkaline Metals
Elements	Li, Na, K	Mg, Ca, Sr
Position	Placed in IA group	Placed in IIA group
Occurrence	Do not occur in free state	Do not occur in free state
Nature	Soft Low melting and boiling point	Hard Greyish white in colour
Bonding	Salts of alkali metals form ionic compounds (except for some lithium salts)	Salts of alkaline metals form ionic compounds (except for beryllium)
Action of Air	React rapidly Reactivity increase down the group	Less reactive Reactivity increases down the group
Action of water	2M + 2H2O → 2MOH + H2	M + 2H2O → M(OH)2 + H2
Action of acids	2M + 2HCl → 2MCl + H2	M + HCl → MCl2 + H2
Ionisation energy	Low ionisation energy	Low ionization energy (but higher than alkali metals)
Colour of the flame	Impart characteristics colour to the flame Crimson red – Lithium (Li) Golden yellow – Sodium (Na) Pale violet – Potassium (K)	Impart colour to the flame Calcium – Brick red Strontium – Crimson Barium – Apple green Mg and Be do not impart colour because the electrons are too strongly bound to be excited.
Obtained	By electrolysis of their molten salts	By electrolysis of their molten salts.

Some other categories of metals

Transition Metals:

• Elements like Fe, Zn etc. are transition metals. They are placed in the middle of the periodic table. 
• Have high melting and boiling point 
• Good conductors of heat and electricity 
• Show variable valencies 

Inner transition metals:

• Elements like La, Ce are inner transition metals. They are placed at the bottom of the periodic table. 
• Heavy metals with high melting and boiling point 
• Good conductor of heat and electricity 
• Show variable valencies 

In the reaction of metals with air, water, and acids, we observed that some metals react very vigorously, some others react rather slowly, and some do not react at all. 

What can you conclude from the given information? Are all elements equally  reactive? 

As different metals react with the same chemicals in different ways, the reactivity of metals cannot be similar. 

If the reactivity of metals is different, then how can we determine the reactivity of  two metals? 

Displacement reactions help us for this. Actually, some metals are more reactive than  others. Metals that are more reactive can displace the less reactive metals from their salts in a solution or molten form. The general equation for such reactions is given as: 

Metal A + Salt solution of metal B → Salt solution of metal A + metal B  

Such reactions are called displacement reactions. In displacement reactions, a more reactive metal replaces a less reactive metal from the latter’s salt. 

For example, iron can replace copper from copper sulphate solution, but copper cannot replace iron from iron (II) sulphate solution. 

When an iron nail is kept in a copper sulphate solution, the intensity of copper sulphate  solution decreases, and the iron nail gets covered with copper.

This means that iron is more reactive than copper as it can replace copper from copper sulphate solution.

The reactivity of metals can be determined by observing their reactions with salt solutions of other metals. When the reactivity of a metal is determined, it can be arranged in an increasing or decreasing order of their reactivities. 

The Reactivity Series

The series in which various metals are arranged in the order of their decreasing  reactivity is called a Reactivity series. 

This series is prepared by performing displacement reactions between various metals and their salt solutions. The reactivity series is given as follows: 

Reactivity series of metals  

• In the reactivity series, metals present above a particular metal are more reactive than that metal, while the metals present below the particular metal are less reactive than it.
• We know that all metals lose electrons to form positive ions. The tendency to lose an electron can be related to the reactivity of metals. If a metal can lose electrons easily,  then it is very reactive. On the other hand, if the tendency of a metal to lose elecrons is less, then it is less reactive. 
• In general, we say that the metals present above hydrogen are more reactive than it, and displace it from acids to liberate hydrogen gas.  
• However, the metals present below hydrogen are less reactive than it, and cannot displace it from acids to liberate hydrogen gas. 
• Metals such as sodium and potassium (that lie above hydrogen) readily react with dilute acids to evolve hydrogen gas, whereas metals such as copper, gold, and silver (that lie  below hydrogen) do not react with dilute acids. 

Main Features of Reactivity Series 

• Metals are arranged in the decreasing order of their electropositive character. 
• Metals at the top have greater reducing power. This power decreases on moving down the series. 
• Metals at the top show greater tendency to get oxidised. 
• Metals above hydrogen in the reactivity series liberate hydrogen gas from mineral acids. 
• Metals at the top displace metals lower in the series from the aqueous solution of their salts. 
• Metal oxides above Al, cannot be reduced by common reducing agents, the  reverse is true for metal oxides below Al. 

Let us now see the action of heat on some metallic compounds like oxides, hydroxide, carbonates and nitrates. 

Oxides: 

Metal	K, Na, Ca, Mg, Al	Zn	Fe, Pb, Cu	Hg, Ag
Action of Heat on metal oxide	Stable to heat Reduced by electrolysis	Reduced by coke only	Reduced by C, CO, H2, NH3	Decompose on heating to give metal and oxygen

Hydroxides:

Metal	K, Na	Ca, Mg, Al, Zn, Fe, Pb, Cu	Hg, Ag
Action of Heat on metal oxide	Stable to heat Soluble in water	Decompose on heating to give metal oxide	Decompose on heating to give metal, oxygen and water vapour

 Carbonates: 

Metal	K, Na	Ca, Mg, Al, Zn, Fe, Pb, Cu	Hg, Ag
Action of Heat on metal oxide	Stable to heat Soluble in water	Decompose on heating to give metal oxide and carbon dioxide	Decompose on heating to give metal, oxygen and carbon dioxide

Nitrates:

Metal	K, Na	Ca, Mg, Al, Zn, Fe, Pb, Cu	Hg, Ag
Action of Heat on metal oxide	Decompose on heating to give metal nitrite and oxygen	Decompose on heating to give metal oxide, nitrogen dioxide and oxygen	Decompose on heating to give metal, nitrogen dioxide and oxygen

Corrosion: Causes and Prevention 

You must have observed that when metals such as iron, silver, and copper are exposed to air for some time, they lose their shine. For example, iron, when exposed to moist air  for a long period of time, acquires a coating of a brown-flaky substance. 

This is because metals react with moisture and the different gases present in the air. The reaction of metals with moisture and gases present in the air is known as  corrosion. Rusting of iron is the most common example of corrosion.

Do You Know? 

Rust is a general term given to iron oxides, which are formed when iron reacts with oxygen in the presence of moisture. Rust primarily consists of hydrated iron (III)  oxides, Fe2O3 .nH2O. The number of water molecules in rust is variable. Hence, they are represented by n. 

Other examples of corrosion: 

1. You must have observed that ornaments made of silver lose their shine after some  time. This is because silver reacts with sulphur present in the air to form silver sulphide,  which forms a layer over its surface. 
2. Copper reacts with carbon dioxide to form copper carbonate, which is greenish in  appearance. This is the reason why a copper article loses its shiny brown surface when  exposed to air. 

Do You Know? 

Corrosion of aluminium metal is extremely slow. This is because of aluminium oxide, which is formed when aluminium reacts with oxygen, is very stable and forms a  protective coating or layer on the surface of the metal. This prevents the oxidation of the  remaining metal. 

Corrosion can drastically reduce the quality and strength of metals. The higher a metal lies in the reactivity series, more readily it is corroded. Here, we will study about the  rusting of iron and the conditions necessary for the same. Let us find out about the  conditions necessary for rusting of iron to take place with the help of the following  activity. 

Therefore, we can say that both air and water are required for rusting to take place. 

Effect of Corrosion on Other Metals based on the Reactivity Series: 

1. Reactive alkali metals react with oxygen, water and carbon dioxide present in air to form  oxide, hydroxide and carbonate, respectively. Hence, they are kept immersed in  kerosene oil to prevent the corrosion. 
2. Aluminium and magnesium when exposed to air form a white layer of the oxide on their  surface. 
3. Iron forms hydrated ferric oxide (rust) on exposure to moisture in the air.
4. Lead forms a white deposit of lead hydroxide and lead carbonate called basic lead  carbonate on coming in contact with moist air. 
5. Copper forms a green deposit of copper hydroxide and copper carbonate called basic  copper carbonate on exposure to moist air. 
6. Silver forms a black coating of silver sulphide on its surface on coming in contact with  hydrogen sulphide present in the air. This phenomenon is known as tarnishing of silver. 

Factors Affecting Corrosion 

Besides oxygen and moisture in the air, there are other factors that enhance the  corrosion of metals. These are: 

1. Reactive nature of metal: Highly reactive metals corrode easily. 
2. Presence of dissolved salts: They act as electrolyte and increase the rate of corrosion. 
3. Presence of pollutants: They increase the rate of corrosion. 
4. Presence of less reactive metal: If a less reactive metal is present, it will make the more  reactive metal susceptible to corrosion. 

Every year our world suffers a huge monetary loss owing to the process of rusting,  which causes harm to articles made of iron. Attempts were made to prevent rusting.  Here are some ways that can prevent rusting or corrosion. 

We now know that both air and water are required for rusting to take place. Thus,  rusting can be prevented by cutting off the contact of iron articles with air or water or  both. There are different methods by which rusting can be checked: 

• Rusting can be prevented by electroplating, painting, oiling, and greasing of iron  articles. In fact, paints and grease should be applied regularly to prevent rusting. 
• Rusting can also be prevented by applying a layer of a metal such as chromium or zinc  on the surface of iron articles. The process of depositing zinc on iron is called  galvanization. 
• Rusting can also be prevented by connecting the iron object with a more reactive metal  like zinc with the help of a wire. The process of connecting iron with a more reactive  metal through a wire is called cathode protection. 
• Alloying can also be used to prevent rusting or corrosion. 

Do you know what alloys are? 

An alloy is a homogeneous mixture of two or more elements, at least one of which is a  metal. An alloy of a metal is made by first melting the metal and then, adding and  dissolving the element with which it is to be alloyed. This is done in a molten state so that an even distribution of elements can take place. Usually, the resulting substance  has properties different from those of its components. 

Alloy	Components	Properties	Uses
Stainless steel	Iron, nickel and chromium	Does not get affected by the action of air, water and alkali.	In preparation of utensils, blades and surgical instruments.
Brass	Copper and zinc	Malleable, strong, corrosion resistant and can be easily shaped.	In preparation of cooking utensils, parts of machines and instruments.
Bronze	Copper and tin	Stronger and more corrosion resistant.	In preparation  of statues, coins and medals.

Do you know?

Pure gold is known as 24 carat gold. In India, the gold that is generally used to make  ornaments is 22 parts of pure gold alloyed with 2 parts of either silver or copper. 

Do you know that alloying is a good method for improving the properties of  metals? 

Properties of metals can be improved by combining them with other elements i.e., by alloying. Alloying can also be used to prevent rusting. Pure iron is not very hard and  stretches when heated. However, when it is mixed with a small amount of carbon, it becomes very hard.  

This is known as steel. Even though steel is hard, it does rust. Stainless steel is obtained when nickel and chromium are added to iron. Stainless steel contains iron as the primary constituent, but it does not rust at all. Thus, by adding different  elements, the properties of iron can be changed.

The iron pillar near the Qutub Minar in New Delhi was made around 400 B.C. It is 8 m tall and weighs around 6 tonnes (6000 kg). The workers who made it knew that pure iron would rust after some time and devised a method to prevent the pillar from rusting. They painted the surface of the pillar using a mixture of salts, followed by heating and quenching (rapid cooling). This finishing treatment resulted in the formation of a thin layer of magnetic oxide (Fe₃O₄) on the surface of the pillar and prevented the iron present in the pillar from rusting.

Even though corrosion and rusting causes much damage, but sometimes this  phenomenon has an advantage. Let us understand with the help of an example.  Aluminium and zinc articles when exposed to air form a white deposit of their respective  oxides on their surface.  

These oxides stick to the surface of the metal and are impervious in nature. So in a  way, this oxide prevents the next layer of metal from getting corroded. This is the  reason why objects made from aluminium and zinc do not corrode easily.  

Concentration of Ores 

Metal	Occurrence
Aluminium	1.Bauxite, Al2O3.x H2O 2. Cryolite, Na3AlF6
iron	1.Haematite, Fe2O3 2. Magnetite, Fe3O4
Copper	1.Copper pyrites, CuFeS2 2.Copper glance, Cu2S 3. Malachite, CuCO3, Cu(OH)2 4. Cuprite, Cu2O
Zinc	1. Zinc blend or Sphalerite, ZnCO3 2. Calamine, ZnCO3 3. Zincite, ZnO

Some Important Terms 

• Metallurgy: Process of extracting pure metal from their ore.
• Minerals: Mixture of metal compounds, soil, sand, limestone and rock.
• Gangue: Impurities present in ore like mud, silica etc. 
• Ores: Minerals from which metals can be extracted economically at low cost and with minimum cost.
• Flux: Substance added in furnace to remove gangue.
• Slag: The fusible mass formed when flux combined with gangue.
• Smelting: Process of extracting metal from their oxide ores by reducing the roasted oxides.

Extraction and Refining of Metals 

Elements on earth are found in different forms from the different parts of the earth: 

• Lithosphere: This part of earth is made up of sand, clay, stone and elements such as  aluminium, copper, iron, calcium, sodium etc. which are found in the form of sulphides  or oxides. 
• Hydrosphere: This part of the earth includes water of seas, lakes and ice of polar  regions. In this part, many non-metals and metals are obtained in combined forms such  as, chlorine, flourine, sodium, potassium, magnesium and calcium.  
• Atmosphere: The blanket of air around the earth is called atmosphere. Non-metallic  gases like nitrogen, carbon dioxide and oxygen are present majorly.  

We know that metals are highly reactive. Therefore, they do not occur in the free state.  For example, sodium, potassium, etc. are never found in the free state and occur in  nature in chemically combined forms known as minerals. 

Elements or compounds, which occur naturally in the Earth’s crust, are known as  minerals. Most minerals found in the earth’s crust contain metals. Some metals are  also found in the oceans in the form of salts such as sodium chloride, magnesium  chloride, etc. 

There are some minerals which contain a large amount of a particular metal and from  them, metals can be extracted profitably (using practically possible techniques). 

The minerals from which metals can be extracted commercially are known as  ores. 

The process by which a pure metal is obtained from its ore is known  as extraction.

Do you know that metals are classified into three groups on the basis of their  reactivity series?

The three groups are as follows: 

1. Metals of low reactivity
2. Metals of medium reactivity 
3. Metals of high reactivity 

Steps Involved in Extraction 

1. Crushing and grinding ore 
2. Concentration of the ore 
3. Roasting and Calcination of the ore 
4. Reduction of the metal oxide 
5. Refining of the pure metal 

Crushing and Grinding 

• Crushing of the ore to fine powder with the help of big jaw crushers and ball mills 
• The process is known as pulverisation 

Concentration (or Dressing or Benefaction) of ores 

• Removal of unwanted materials such as sand and clay from ores 

Some Important Procedures 

Hydraulic washing 

• Based on gravity differences between the ore and the gangue particles 
• In one such process, the lighter gangue particles are washed away by a stream of  water, leaving behind the heavier ore. 

Magnetic separation 

• Based on differences in magnetic properties between the ore and the gangue particles 
• Magnetic field is applied to separate magnetically attractive particles from magnetically  non-attractive particles. 
• Schematic diagram of magnetic separation is as follows:
  

Froth-Floatation method 

• Applied to remove gangue from sulphide ores 
• Mineral and gangue particles are separated by first wetting the mineral particles with oil,  and gangue particles with water, and then the mineral particles are carried out by  forming froth. 
• Sometimes, depressants are used for separating two sulphide ores by selectively  preventing one ore from forming froth. For example, NaCN is used as a depressant for  separating two sulphide ores, ZnS and PbS. 

Leaching 

Used if the ore is soluble in some suitable solvent.

• Leaching of alumina from bauxite:
  
  2Na[Al(OH)₄]_(aq) + CO_2(g) → Al₂O₃.xH₂O_(s) + 2NaHCO_{3 (aq)}
  
• Leaching of some other ores: Ores of metals like Ag and Au are leached with a dilute solution of NaCN and KCN in  the presence of air.
  4M_(s)+ 8CN^-_(aq) + 2H₂O_(aq) + O_2(g) → 4[M(CN)₂]^-_(aq) + 4OH^-_(aq)
  (M = Ag or Au)
  2[M(CN)₂]^-_(aq) + Zn_(s) → [Zn(CN)4]^2-_(aq) + 2M_(s)

Roasting and Calcination of the Ore

Metals such as mercury and copper, which lie quite low in the reactivity series, exist in  nature as sulphides. These ores when heated react with oxygen present in the air and  get converted into oxides. When these oxides are further heated, pure metals are obtained. The process in which a sulphide ore is heated in the presence of air is known  as roasting. 

Metals of medium reactivity i.e., metals that are present in the middle of the reactivity  series such as zinc, iron, lead, and manganese are quite reactive and exist in nature as  oxides, sulphides, and carbonates. These metals are extracted from their ores by first  converting ores to oxides and then by the reduction of these oxides, mostly using  carbon. 

There are two methods by which ores are converted into their respective oxides: 

1. Roasting 
2. Calcination 

Roasting: It is used to convert sulphide ores into oxides. Roasting involves strong heating of iron ore in the presence of excess air. For example, copper sulphide in  copper glance ore is converted into copper (I) oxide by heating it in the presence of  oxygen. 

Calcination: It is used to convert carbonate ores into oxides. Calcination involves strong heating of the ore in the absence of air. For example, calamine ore, which is chemically zinc carbonate, is converted into zinc oxide by heating it in the absence of air.

After obtaining metal oxides from the ores, reduction of these metal oxides is done to obtain pure metals. Mostly, carbon in the form of coke is used for this.

However, the oxides of metals, which are present relatively higher in the reactivity  series such as manganese, cannot be reduced with coke. To reduce these oxides of metals, more reactive metals than manganese such as sodium, calcium, and  aluminium are used.  

For example, iron is also a very reactive metal and cannot be reduced using carbon. Hence, it is reduced using aluminium metal. The reaction is highly exothermic. The heat evolved is so large that the metals are produced in the molten form. This reaction is known as thermite reaction and is used to join railway tracks or cracked machine parts.

Similarly, manganese cannot be reduced using carbon. Hence, it is also reduced by aluminium metal. 

Metals present at the top of the series such as sodium, potassium, calcium,  manganese, and aluminium are very reactive. These metals cannot be reduced using coke as their affinity for oxygen is much more than that of carbon. Therefore, these  metals are reduced by passing an electric current through their molten salts. This process is known as electrolytic reduction. 

For example, sodium metal is extracted from sodium chloride. To extract the metal,  electrolytic reduction of molten sodium chloride is carried out. When an electric current  is passed, sodium ions which have positive charge move towards the cathode and get  deposited over it after accepting electrons. The chloride ions have a negative charge  and move towards the anode, lose their extra electrons, and escape out of the solution  as chlorine gas. 

Reaction at the cathode (negative electrode): Na⁺ + e^- → Na

Reaction at the anode (positive electrode): 2Cl^- → Cl₂ + 2e^-

Do you know that the metals obtained by various reduction processes, except  electrolytic reduction, contain many impurities and require purification? How are  these metals purified? 

The method that is most commonly used to purify metals is electrolytic refining.  Many metals such as copper, zinc, gold, etc. are refined electronically. 

Refining:

This method is used for the metals with low melting points i.e, which melt easily. A furnace with a slope in it, temperature is kept slightly higher than the melting point of the  metal. When the impure metal is passed through the furnace, the pure metal is melted  there and collected in the vessel. However, the melting points of the impurities is higher  than the the metal so that they can be found solid on the slope. 

Zone Refining:

This method works on the principle of fractional distillation and trace impurities are  removed from the metal using this method. The impurities remain more soluble in  molten form which upon cooling the molten metal, decreases the solubility of impurities  and separates in the from crystals. Semi-metals such as boron, silicon are refined by  this method. 

Refining 

Distillation 

• Involves evaporation of impure metal 
• Used for metals having low boiling points, such as Zn, Hg 

Liquation 

• Involves flowing of low melting metal like tin on a sloping surface so that higher melting  impurities are left behind 

Electrolytic Refining 

• Impure metal is refined using electricity. 
• The impure metal is made the anode, and a strip of pure metal is made the cathode. 
• A solution of a soluble salt of the same metal is taken as the electrolyte. 
• Impurities get collected below the anode and are known as anode mud. 
• At anode:  M → Mn⁺ + ne^-
• At cathode: Mⁿ⁺ + ne^- → M

• Anode mud of blister copper contains antimony, selenium, tellurium, silver, gold and  platinum. 

Zone Refining 

• Principle − Impurities are more soluble in the molten state of metal (the melt) than in the  solid state. 
• In this process, a circular heater is fixed at one end of a rod of impure metal.

• As the heater moves, the molten zone of the rod also moves along with it. 
• As a result, pure metal crystallises out of the melt, and the impurities pass to the  adjacent molten zone. 
• This process is repeated several times, which leads to the segregation of impurities at  one end of the rod. Then, the end with the impurities is cut off. 

Vapour Phase Refining 

In this process, the impure metal is converted into its volatile compound, which is  decomposed to obtain the pure metal. 
To carry out this process:

• The metal should form a volatile compound with an available reagent 
• The volatile compound should be easily decomposable so that the metal can be easily  recovered 
• Nickel, zirconium and titanium are refined using this process 

• Mond process for refining nickel 

• van Arkel Method for refining zirconium or titanium 

• All the oxygen and nitrogen present as impurity are removed. 
• Crude metal is heated with iodine in an evacuated vessel.
  Zr   +   2I₂   →     ZrI₄ 
• Metal iodide is decomposed on a tungsten filament, electrically heated to about 1800 K  to give the pure metal.
  ZrI₄     →    Zr    +   2I₂

Chromatographic Methods 

Principle − Different components of a mixture are differently adsorbed on an adsorbent. • Some chromatographic techniques are − 

• Column chromatography 
• Paper chromatography 
• Gas chromatography 

There are two phases in chromatography: mobile phase and stationary phase. 

• Column chromatography is useful for the purification of elements available in minute  quantities. It is also used for removing the impurities that are not very different in  chemical properties from the element to be purified.
• Adsorbed components are removed (eluted) using suitable solvents (eluents). 
• Schematic diagrams of column chromatography in industrial and laboratory methods  are as follows:
  

Metallurgy of Aluminium 

Some Common Ores of Aluminium 

1. Bauxite or hydrated aluminium oxide ((Al₂O₃.H₂O) 
2. Cryolite or sodium aluminium fluoride (Na₃ AlF₆) 
3. Corundum or anhydrous aluminium oxide (Al₂O₃) 

In the metallurgy of aluminium, first, the ore of aluminium is concentrated. 

Removal of unwanted materials such as sand and clay from ores is known as concentration (or dressing or benefaction) of ores. 

Aluminium is generally extracted from the bauxite ore. The bauxite ore can be  concentrated by the process of leaching. 

Leaching of alumina from bauxite (Baeyer's Process) 

The powdered ore of bauxite is concentrated by digesting it with concentrated sodium hydroxide solution at 473-523 K and 35-36 bar pressure. As a result, Al₂O₃ is leached out as sodium aluminate along with SiO₂ as sodium silicate leaving behind the impurities.

Then, carbon dioxide gas is passed through the aluminate solution to neutralise it and as a result, hydrated Al₂O₃ gets precipitated. Freshly prepared hydrated Al₂O₃ is added to induce the precipitation.

 2Na[Al(OH)₄]_(aq) + CO_2(g) → Al₂O₃.xH₂O_(s) + 2NaHCO_3(aq)

The hydrated Al₂O₃ is filtered out of the solution, then dried and heated to obtain pure Al₂O₃ while sodium silicate remains in the solution. 

Another method of extracting alumina from bauxite is the Hall's process. 

Hall's Process 

In this method the ore is powdered and heated with an aqueous solution of sodium  carbonate. This results in the formation of water soluble sodium aluminate. The  insoluble impurities are filtered out and the filtrate is warmed and neutralised by passing  carbon dioxide gas through it. This causes the precipitation of aluminum hydroxide. 

Al₂O₃.2H₂O(s) + Na₂CO₃ → 2NaAlO₂ (aq) + CO₂ + 2H₂O (l)

2NaAlO₂ (aq) + 3H₂O + CO₂ (g) → 2Al(OH)₃ + Na₂CO₃

Alumina which is obtained after Bayer's process and Hall's process is very stable and  cannot be reduced using conventional reducing agents. Aluminium can be extracted  from the pure alumina by the process of electrolytic reduction. 

Electrolytic Reduction of Alumina

Electrolytic cell for the extraction of aluminium 

The melting point of alumina is too high to be used as an electrolyte. In this process, 60% of Na₃AlF₆ with 20% of CaF₂ is added to 20 % of pure alumina to lower the melting point and to increase the conductivity of the electrolyte. The inner lining of the electrolytic cell acts as the cathode and graphite rod acts as the anode. 

The temperature of the electrolyte is maintained at 950^oC by electrical heating using a  voltage of 5 to 6 V. This low voltage avoids decomposition of molten cryolite. 

The overall reaction that takes place is 

2Al₂O₃ + 3C → 4Al + 3CO₂

This electrolytic process is known as the Hall-Heroult's process. The oxygen liberated at  the anode reacts with the carbon of the anode to produce CO and CO2. Therefore, the  carbon anode has to be replaced periodically as the oxygen released oxidises it. 

The cell reactions are: 

At Cathode: Al³⁺_(melt) + 3e^- → Al_(l)

At Anode:

2O^2- → 2O + 4e^-

O + O → O₂

C _(s) + O^2-_(melt) → CO (g) + 2e^-

C _(s) + 2O^2- _(melt) → CO₂(g) + 4e^- 

The anode is oxidised by the oxygen-evolving at anode so it has to be replaced from  time to time. 

Functions of Important Substance in Extraction of Aluminium 

Cryolite: 

1. Lowers melting point 
2. Increases conductivity 

Fluorspar and cryolite: 

1. Solvent for electrolytic mixture 
2. Increases conductivity

Powdered coke: 

1. Reduces heat loss by radiation 
2. Prevents burning of anode 

Purification of aluminium 

Aluminium obtained is about 99.9 % pure. However, it can be further purified by an  electrolytic process known as Hoope’s process. In this process, a three-layered  electrolytic cell is used. These layers are made up of molten substances of different  specific gravity. 

Bottom layer: Molten impure aluminium with Carbon lining (anode) 

Middle later: Mixture of sodium fluoride, barium fluoride and aluminium fluoride 

Top layer: Molten pure aluminium with carbon electrodes (cathode) 

When current passes through the cell, the aluminium from middle layer moves to the top  and similarly, an equivalent amount of aluminium from bottom layer moves to the middle  layer. Pure aluminium is thus obtained from the top layer. 

Reactions:

Physical and Chemical Properties 

Physical Properties 

• Appearance: Silvery white metal 
• Strength: Light but strong metal with high tensile strength 
• Malleable and ductile 
• Good conductor of heat and electricity 
• Lustrous after polishing 
• Melting and boiling point: 600 ^oC and 2050 ^oC, respectively 

Chemical Properties 

• Reaction with air:
  
  2Al + N₂ → 2AlN
• Reaction with water:
  2Al + 3H₂O (steam) → Al₂O₃ + 3H₂
• Atmospheric nature:
  2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂
  2Al + 6 HCl → 2AlCl₃ + 3H₂
• Reaction with non-metals
  
  
  
  
• Reducing nature : Aluminothermy
  Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + Heat

Uses of aluminium 

Some important uses of aluminium are: 

• As wrappers for foods in the form of foils 
• In paints and lacquers 
• In the extraction of chromium and manganese from their oxides 
• In conduction of electricity in the form of wires 

Alloys of aluminium are also useful as they are light.

Some alloys of aluminium are: 

• Duralumin (95 % Al, 4 % Cu, 0.5 % Mg, 0.5 % Mn) which is used in the manufacture of  aeroplanes as it is very light, strong, ductile, and resistant to corrosion 
• Magnelium (90 % Al, 10 % Mg) which is used in the manufacture of light tools and  machine parts as it is very light, strong, and resistant to corrosion.

Uses of Metals and Non-Metals 

We are familiar with a number of substances, which are very hard and shiny in nature  such as iron, aluminium, gold, silver, and copper. You must have observed that these materials produce a sound on being struck. Such substances are called metals.

Substances which are dull in appearance and not very hard are called non-metals such as carbon, sulphur, iodine, etc. 

There are 92 naturally occurring elements, which are classified into metals and non metals. Among them, most elements are metals with less than 20 elements as non metals. Here, we will discuss the properties and uses of metals and non-metals. Metals are hard and shiny in appearance. They are malleable, ductile, and good conductors of heat and electricity. As a result of all these properties, metals have many uses.

1. Metals such as gold and silver are very shiny in appearance. These metals are quite  ductile and malleable in nature. Also, these metals are expensive and do not corrode  easily (though silver becomes black after some time due to corrosion). Hence, these  metals are used in making jewellery. 
2. Metals such as copper and aluminium are used to make wires as they are very good  conductors of electricity. Also, they are very ductile. Copper and aluminium wires are  widely used in electrical fittings in houses. 
3. Metals such as iron, copper, and aluminium are good conductors of heat. Hence, they are used for making cooking utensils and water boilers. 
4. Metals are malleable. Hence, they can be hammered into very thin sheets. For example, silver and aluminium foils are made by hammering these metals. Silver foils  are used for decorating food items, whereas aluminium foils are used for wrapping food  items such as chocolates and many such materials. 
5. Metals are hard and rigid. Hence, they can be used in making machinery, automobiles, aeroplanes, trains, satellites etc. Aluminium is used for making parts of  aeroplanes as it is very light in comparison to other metals. 

Do You Know: 

Silver is shiny and is a good reflector. It reflects about 90 percent of light falling on it. Hence, it is used for making high reflecting mirrors. 

Like metals, non-metals also have various uses. We will now discuss the uses of non metals. 

1. Oxygen, which is a non-metal, is essential for life. It is used by plants and animals for  the process of respiration. Oxygen is also used in factories, homes etc. as it supports  combustion. 
2. Nitrogen, a non-metal, is used in fertilizers to enhance the growth of plants.
3. Chlorine has the ability to kill germs. Hence, it is used in water purification as a  disinfectant. 
4. Tincture iodine is a solution of iodine in alcohol, which is used as an antiseptic.
5. Non-metals are also used in manufacturing crackers. 

Some Common Uses of Metals 

Uses of aluminium 

• Aluminium is cheap and resistant to corrosion, so it is used for making cooking vessels,  picture frames and household fittings. 
• It is used in high-voltage electric transmission wires. 
• Aluminium foils are used for packing purposes. 
• It is used for making alloys like duralumin and magnalium. 
• It is also used in paints. 
• It is used in making mirrors of telescopes as it is an excellent reflector of light. 
• It is used in thermite welding. 
• Thermit (a mixture of 3 parts of Fe₂O₃ and 1 part of Al powder) is covered with an ignition mixture (Potassium chlorate and magnesium powder) in a crucible. 
• The ignition mixture is ignited using a fuse of burning magnesium. 
• In the reaction, Fe₂O₃ is reduced to Fe with the evolution of a large amount of heat.
• The molten Fe falls between the broken pieces and solidifies, joining the pieces in turn.
  Fe₂O₃ + 2 Al → Al₂O₃ + 2 Fe + Heat
• Aluminium-air batteries, also called Al-air batteries, are batteries in which the  reaction of oxygen present in the air with aluminium is used to produce electricity.

Uses of magnesium

• Magnesium is a silvery white metal. 
• A mixture of powdered magnesium and potassium chlorate is used in fireworks. • It is used as a fuse wire in thermite welding. 
• It is used as a reducing agent in the extraction of metals. 
• It is also used for the preparation of alloys like magnalium. 

Uses of mercury 

• It is used as a thermometric liquid in labs. 
• It is used in thermometers. 
• It is also used as an amalgam in dentistry for filling tooth cavities. 

*As a liquid mirror, mercury is used as an alternative to big telescopes.

Uses of zinc 

• It is used for galvanising iron. 
• It is used for making containers of the dry cell. 
• It is used in the preparation of alloys. 
• It is also used in the extraction of gold and silver. 

*The most exploited zinc ore is sphalerite or zinc sulfide; the largest exploitable deposits  are found in Germany, Canada and the United States

Uses of iron 

• Wrought Iron (carbon content 0.1 - 0.25%) is used for making tin roofing, buckets,  trunks and electromagnets. 
• Cast iron (carbon content 2.5 - 5%) is used for making drain pipes, manhole covers and  machinery. 
• It is also used for manufacturing steel.

Uses of copper 

• It is used for making electric transmission wires. 
• It is used in the coils of electric motors and electric generators. 
• It is used for making alloys such as brass and bronze. 
• It is used in the radiators of automobiles. 
• It is also used for making coins and printed circuits.
  

Some Common Uses of Non-metals 

Uses of Hydrogen

It is the lightest element. It is found in the gaseous state. 

• It is used as a non-polluting fuel. It is present in coal gas and water gas. 
• Oxy-hydrogen flame is used for cutting and welding metals. 
• It is also used for filling weather observation balloons.
  

Uses of Nitrogen 

• It dilutes the activity of oxygen, so it is used for controlling the rate of combustion. 
• It helps plants manufacture proteins.
• It is used in the manufacture of ammonia gas. 
• It is also used for preserving packaged food.
  

Uses of Oxygen 

• It is essential for the respiration of living beings. It is also needed for artificial respiration. 
• It is required for the combustion of fuels and is also used in rocket fuels. 
• As dissolved oxygen, it keeps water fresh and is used for respiration by marine  organisms. 
• It is also used for cutting and welding purposes. 

*The diamagnetic form of molecular oxygen (O₂) is commonly known as molecular  oxygen.

Uses of Chlorine 

• It is used in bleaching powders. 
• It is used for sterilising drinking water. 
• It is also used in pesticides and acids. 

*Insecticides and pesticides are used for killing insects. They include fungicides,  larvicides and rodenticides.

 Uses of Iodine

• In the form of sodium iodide or potassium iodide, it is required for the proper functioning  of the body. 
• In the form of silver iodide, it helps in making photographic films. 
• It is also used for dressing wounds. 
• In the form of iodoform, it is used in medicines. 

*Iodoform is a compound of iodine with the chemical formula CHI₃. It is a pale-yellow  solid which was quite commonly used in antiseptics and disinfectants.

 Uses of Carbon 

• It is used in the electrodes of electrolytic cells. 
• In the form of graphite, it is used as a dry lubricant, and as pencil lead. 
• Graphite is also used as electrode material in electrolytic cells because it is a good  conductor of electricity. 
• It is used for making heat-resistant crucibles. 
• It is employed in nuclear reactors. 
• It is used in carbon arc lamps. 
• Coal is used as a fuel in homes, industries, pharmaceutical and textile sectors. 
• Diamond is the most crystalline form of carbon and is used as a precious gem.  The impure gem is used for grinding hard substances and drilling heads. 

*Coke is the dry solid material left after heating coal to a very high temperature.

Uses of Sulphur 

• It is used in the chemical industry for manufacturing sulphuric acid, sodium thiosulphate,  carbon disulphide, etc. 
• It is used in insecticides and fungicides 
• It is used in medicines. 
• It is also used for vulcanising rubber.

* Natural rubber is sticky, easily deforms when warm and is brittle when cold. Vulcanisation refers to a specific process which involves heating rubber to high temperatures and adding sulphur or other equivalent curatives.

Some Common Uses of Metalloids  

Uses of Silicon 

• It is used for making solar cells, microchips and transistors. 
• It is used for manufacturing polymers, also called silicones. 
• It is used for manufacturing ferro-silicon, a special form of steel and silicon carbide. It is  one of the hardest substances known. 
• It is a very important component of cement and glass. 

*A solar cell or photovoltaic cell is a device that converts light into electric energy.

Uses of Germanium 

• Germanium is commonly used as a semiconductor. 
• It is used as a transistor in many electronic applications when mixed with arsenic,  gallium, etc. 
• It is used to form alloys and as a phosphor in fluorescent lamps. 

Noble gases 

• Noble gases are very non-reactive gases and are therefore used to provide the inert  environment. 
• Helium: for filling weather observation balloons 
• Argon: For filling electric bulbs 

The metals that are not acted upon by mild acids and alkalis, and occur in nature in the  free state are called noble metals. Thus, they are resistant to corrosion and oxidation.  These metals are very precious. 

They include − 

• Silver 
• Gold 
• Platinum
• They also include ruthenium, rhodium, palladium, osmium and iridium.
  

*In India, pure gold is denoted as 24 carats. The gold that is generally used for making  ornaments is 22 parts of pure gold alloyed with 2 parts of either silver or copper. This  mixture is known as 22 carat gold.

Uses of silver 

Silver is a shiny, heavy metal, and the best conductor of electricity. 

• It is used for making silver ornaments and expensive utensils such as glasses, mugs,  etc. 
• It is used for making coins. 
• Salts of silver like silver chloride are used for making photographic films. 
• Silver foils are used for decorating sweets. 
• Silver is also used for making mirrors using a process called sputtering. 

Uses of gold 

Gold is bright yellow and a highly malleable and ductile metal. 

• Gold is used as the index of wealth. The countries which have more gold reserve are  considered to be wealthy. 
• It is used for making ornaments. N 
• It is used for making high-value coins and medals.
• It is used for covering the mainframe of artificial satellites. 

Uses of platinum 

Platinum is silvery white, a highly malleable and ductile metal. 

• It is used for making ornaments and watches. 
• It is used as a catalyst in the manufacture of sulphuric acid and nitric acid. 
• It is used in platinum catalytic converters. 
• It is also used in chemical laboratories. 

*The word ‘platinum’ has been derived from the Spanish term platina del  Pinto.

Alloys

Alloys are homogeneous mixtures of two or more elements (at least one of which is  metal). They are made to improve the properties of metals such as their malleability,  ductility, strength, and hardness. 

Purpose of making alloys: 

(a) To change the property of metal 

(b) To achieve a specific objective 

Reason to make alloys: 

The process of alloying the metals alters their properties such as: 

(a) Enhanced appearance 

(b) Altered chemical reactivity 

(c) Lowered melting point 

(d) Modified casting ability 

(e) Increased hardness 

(f) Enhanced tensile strength 

(g) Increases electrical resistant 

Characteristics of an alloy: 

(a) It enhances the hardness of metals. 

(b) It increases the tensile strength of metals. 

(c) It improves the corrosion resistance of metals. 

(d) It changes or modifies the colour. 

(e) It improves the castability of metals.

How do we prepare alloys? 

Alloys are obtained by melting two or more elements in fixed proportions and then  cooling them to room temperature. 

We will now discuss some common alloys. 

1. Stainless steel 

Stainless steel is obtained by combining carbon, chromium, and nickel in iron. The  composition of various elements in steel is: 

Fe(73%) + Cr(18%) + Ni (8%) + C(1%) 

Stainless steel has many advantages over iron. The most important is that unlike iron, it  does not rust. Hence, it is widely used for making utensils, cutlery, surgical instruments,  and ornamental articles. It is also stronger than iron. 

2. Alloyed gold 

Did you know that although gold is a very soft metal, it has a very high melting point of  1064ºC? Therefore, it is very difficult to work upon. Metals such as silver or copper are  added to gold in small quantities to make it harder. 

3. Duralumin 

Duralumin is obtained by combining copper, manganese, and magnesium in aluminium.  The composition of various elements in duralumin is: 

Al (95%) + Cu (4%) + Mn (.5%) + Mg (.5%) 

It is very lightweight, yet very hard and strong. Therefore, it is used for making frames of  aircrafts, automobiles, and speedboats. 

It is also used for making household articles. 

4. Brass 

Brass is obtained by mixing zinc and copper. The composition of various elements in  brass is: 

Zn (40-30%) + Cu (60-70%) 

It is used for making electric switches, statues, utensils, and ammunition.

5. Bronze 

Bronze is obtained by mixing tin and copper. The composition of various elements in  bronze is: 

Cu (80%) + Sn (18%) + Zn (2%) 

It is very resistant to corrosion. Therefore, it is used for making coins, statues, and  utensils. 

Alloys of iron and zinc 

Alloys are homogeneous mixtures of two or more metals. Alloys are prepared to  enhance the properties of metals. 

Some alloys of iron are steel, stainless steel, tungsten steel, nickel, chrome steel, etc.  Steel contains carbon. The constituents of stainless steel are Fe, C, Cr, and Ni. It is  used in automobiles, cycles, pens, utensils, etc. Tungsten steel, which contains 20%  tungsten, is used in high speed machinery.  

Nickel steel, which contains 36% nickel, is used for making cables, automobiles,  aeroplane parts, pendulum, measuring tapes, etc. Chrome steel is used for cutting tools  and crushing machines. 

The table given below discusses some more important alloys:

Primary Metal	Name of the Alloy	Composition of the Alloy	Properties of the Alloy	Uses of the Alloy
Aluminium	Magnalium	Al (90 – 95%) Mg (10 – 5%)	(a) Corrosion resistant (b) Light and strong	(a) Making aircrafts (b) Making scientific instruments (c) Making mirrors (d) Making household appliances
Aluminium	Alnico	Al, Ni, Co, Fe	(a) Corrosion resistant (b) Light and shiny	(a) Making magnets
Iron	Manganese steel	Fe (85%) Mn (14%) C (1%)	(a) Durable (b) Hard	(a) Making safes (b) Making rock drills (c) Making armour
Iron	Tungsten steel	Fe (84%) W (5%) C (1%)	(a) Hard	(a) Making cutting tools for high-speed lathes
Iron	Nickel steel	Fe (95% - 98%) Ni (5 – 3%)	(a) Hard (b) Elastic (c) Corrosion resistant	(a) Making electric wires (b) Making scientific equipment
Iron	Invar	Fe (63%) Ni (36%) C (1%)	(a) Negligible expansion	(a) Making metre scale (b) Making scientific equipment
Copper	German silver	Cu (50%) Zn (30%) Ni (20%)	(a) Silvery light alloy (b) Malleable (c) Ductile (d) Electricity resistant	(a) Making decorative items (b) Making electric heaters (c) Making rheostat (d) Making resistors
Copper	Bell metal	Cu (78%) Sn (22%)	(a) Hard (b) Brittle (c) Easily cut	(a) Making barrels of canons (b) Making bearings (c) Making gears
Lead	Solder (Fuse metal)	Pb Sn	(a) Low melting point (b) High tensile strength	(a) For welding purposes (b) For making fuse
Lead	Type metal	Pb (75%) Sb (15%) Sn (10%)	(a) Low melting point (b) Easy to cast	(a) For making printing  blocks

Some people think that alloys were the first discovery of modern science. But do you  know that alloys are not at all a discovery of modern science? 

History of alloys 

1. The first alloy created by man was bronze. It got its name from the Italian word ‘bronzo’ or the Persian word ‘birinj’. Bronze, an alloy of copper and tin, came into use from as early as 3000 
2. In the third millennium BC, the Sumerians developed bronze to make tools and weapons for ruling their neighbours. 
3. In Asia, some mines produced good quality alloys that were used to make better musical instruments and mirrors. In the 16th century BC, Persians developed carbon steel and started the Iron Age. 
4. In the 20th century BC, the Romans developed their own gold substitute. Under the reign of Caesar Augustus, they developed brass, containing 75% copper and 25% zinc. About 125 years ago, white gold was developed. This alloy was called German Silver or Mock Platinum. In the 12th century, silversmiths in Germany started using Sterling Silver alloy for coinage. 
5. The first official coinage system of Lydia was developed by King Croesus during the period 560–546 BC, using a naturally occurring alloy of gold and silver called electrum.

On the basis of composition, alloys are of two types. They are as follows:

(a) Substitutional alloy: In this kind of alloy, an atom of one metal randomly replaces the atom of the other.

(b) Interstitial alloy: In this kind of alloy, small atoms of elements like hydrogen, boron, carbon and nitrogen occupy the holes in the crystal structure of the metal. 

On the basis of their constituents or elements, alloys can be of two types. They are as follows: 

(a) Ferrous alloy: Alloys which contain iron as the base metal are known as ferrous alloys. For example steel, alnico (used for making magnets), etc.

(b) Non-ferrous alloy: Alloys which does not contain iron as the base metal are known as non-ferrous alloy. For example brass, bronze, duralumin, etc. 

Uses of different alloys: 

Bell metal	Copper : 77%	Casting of bells
Duralumin	Aluminium : 95% Copper : 4% Manganese : Less than 1% Magnesium : 0.5 %	Aircraft parts, boats, railroad cars, ships and nails
Brass	Mainly copper Zinc : Up to 50%	Hose nozzles, screws, inexpensive jewellery, window and door fittings
Bronze	Mainly copper Tin : Up to 12%	Coins, medals, heavy gears, statues, machine parts
Solder	Lead : 50% Tin: 50%	In electrical and plumbing industries to join two metals together
Steel	Iron, carbon, chromium, nickel and tungsten	Cooking utensils, household articles, constructions of bridges and buildings
Sterling silver	Silver : 92.5 % Copper : 7.5 %	Jewellery, art objects
Alnico	Iron, aluminium, nickel and cobalt	Magnets which are much stronger than ordinary magnets

Do you know? 

Certain elements are added to steel to enhance its properties. Some of them are mentioned in the table below.

Alloying Agent	Property	Use
Nickel	It increases strength and hardness. It also makes steel resistant to corrosion.	Turbine blades, engine parts, etc.
Chromium	It resists wear and tear and enhances its resistance to corrosion. It also enhances the hardness and toughness of steel.	This alloy is known as stainless steel. It is mainly used for making kitchen utensils and surgical instruments.
Vanadium	Increases strength and toughness of steel, including its resistance to wear and tear.	Crankshaft, hand tools, surgical instruments, etc.