Pure Metal

Pure metals have the following physical properties:

(a) High density

(b) High melting and boiling points

(c) Good conductors of heat and electricity

(d) Malleable

(e) Ductile

(f) Lustrous

The properties of a pure metal are reflected by its arrangement of atoms.

Pure metal is made up of one type of atoms,thus all atoms are of the same size.

In the solid state,the atoms in a pure metal are orderly arranged and closely packed

together.Thus,pure metals have high densities.

Atoms in pure metal are orderly arranged and closely packed together.

The atoms in metal are orderly arranged in layers to form a three-dimensional crystal

lattice.

The forces of attraction between the very closely-packed atoms are very strong,thus alarge

amount of energy is required to overcome those forces.As a result,pure metals have high

boiling points

Although the forces of attraction between the metal atoms are very strong,they are not

rigid.Therefore,when a forceis applied,the layers of atoms can slide over one

another.Thus,metals are ductile or can be stretched.

Force

Force

Layers of atoms slide over one another

The arrangements of atoms in pure metals are not perfect.There are some empty spaces

between the atoms.When a metal is knocked or pressed,groups of atoms may slide and

then settle into new positions.This explains why metals are malleable or can be shaped.

Pure metals are weak and soft due to their ductility and malleability..Thus, pure metals

have limited uses.

To improve the properties of a pure metal, it is made into an alloy.

Alloys

o An alloy is a mixture of two or more elements with certain fixed composition in which the

major componrnt is a metal.

o Most alloys are mixtures of metals.For example, bronze is an alloy of copper and tin.Both

pure copper and tin are soft.When copper is alloyed with tin, bronze which is harder and

stronger is produced.

o Some alloy smay containvmixtures of a metal and a non-metal such as carbon.For example,

steel is an alloy of iron and carbon.Iron is a soft metal.When some carbon is added to iron,

steel which is stronger and harder is formed.

o Pure metals are normally soft and easily oxidised.This is the reason why monuments or

statues are made of bronze (an alloy) and not copper (a pure metal).

o Cutlery is made of stainless steel and not steel or iron.This is because stainless steel is

shiny and does not rust.

o Alloys are stronger, harder, resistant to corrosion, have a better finish and lustrous.

o By changing the percentage composition of the metals, the properties of the resulting alloy

can be altered.

Why make alloys?

The aim of making alloys are:

(a) To increase the strength and hardness of a pure metal

(b) To increase the resistance to corrosion of a pure metal

(c) To improve the appearance of apure metal

To increase the strength and hardness of a pure metal

(a) In the process of making alloys, atoms of other elements are added, usually in small

amounts into a molten pure metal.When the metal becomes solid again, the positions of

some of the atoms of the pur metal are replaced by the atoms of other elements of different

sizes.

(b) The presence of the atoms of other elements distrupts the orderly arrangement of the pure

metal.The layers of metal atoms are prevented from sliding over one another easily.This

makes alloys stronger and harder than pure metals.

(c) During the making of the steel, carbon atoms which are smaller than iron atoms are added

into iron atoms.As a result, the uniformity of the arrangement of iron atoms is distrupted

and it is more difficult to for the layers of the iron atoms to slide over one another.This

makes steel harder and stronger than pure iron.

To increase the resistance to corrosion of a pure metal

(a) Unreactive metals such as gold and silver can be found in the free state.This is because

they do not react with oxygen and water vapour in the air.

(b) Most metals such as iron and copper corrode readily in the air.

(c) Alloying can prevent metals from corrosion.This is because alloying helps to prevent the

formation of oxide layer on the surface of the metal.

(d) For example, carbon, chromium and nickel are added to iron to make stainless

steel.Cutlery made from stainless steel does not corrode.

To improve the appearance of a pure metal

(a) Metals have lustrous surfaces.However, the formation of dull metal oxide on the surface of

a metal makes it quickly lose its shine.

(b) Alloying helps to keep the metal surface shiny as it prevents the formation of the metal

oxide.

(c) For example, atoms of antimony and copper are added to tin to make pewter.Pewter has a

more lustrous surface than tin.

Composition, properties and uses of alloys.

Bronze was discovered 6000 years ago by mixing copper with tin.Bronze was so widely

used for many years that the period of 3500 – 1200 BC is called the Bronze Age.

Today, many alloys have been discovered and are being improved by altering their

percentage composition.

Alloys such as bronze, brass stainless steel, duralumin and pewter are commonly used in

our daily life.The uses of each different type of alloys depend on the properties of the

alloy.

The table below shows the compositon, properties and uses of some alloys.

Alloy Composition Properties Uses

Bronze 90% copper,

10% tin.

Hard, strong, does not

corrode easily, shiny surface.

Medals, statues, monuments,

art objects.

Brass 70% copper,

30% zinc.

Harder than copper Musical instruments,

kitchenware, door knobs,

bullet cases, electric parts,

ornaments

Cupro-nickel 75% copper,

25% nickel.

Beautiful surface, shiny,

hard, does not corrode easily

Coins

Steel 99% iron,

1% carbon.

Hard, strong Buildings, bridges, body of

cars, railway tracks

Stainless steel 74% iron,

8% carbon,

18% chromium.

Shiny, strong, does not rust Cutlery, surgical instruments,

sinks pipes

Duralumin 93% aluminium,

3% copper,

3% magnesium,

1% manganese.

Light, strong Body of aircrafts and and

bullet trains

Pewter 96% tin, Shiny, strong, does not Art objects, souvenirs

3% copper,

1% antimony.

corrode

9-carat gold 37.5% gold,

11% silver,

51.5% copper.

Shiny, strong, does not

corrode

Jewellary

Composition, properties, and uses of some alloys

What are polymers?

The term ‘polymer’ originated from Greek words, ‘poly’ means ‘many’ and ‘mer’ means

‘parts’.

Polymers are large long-chain molecules formed by joining together many identical

repeating sub-units called monomers.

Polymerisation is a process by which the monomers are joined together into chain-like

molecule called polymer.

M+M+M+M+M+M+M+ … -M-M-M-M-M-M-M

or

nM -( M )- n

where M = Monomer , n = a big number

A polymer may consist of thousands of monomers.

Polymers can be divided into two types :

(a) Natural polymers

They exist in living things in nature.For examples, protein, cellulose, wool, silk, starch,

natural rubber and DNA.

(b) Synthetic polymers

They are man-made in laboratory trough chemical processes.For examples, plastics and

nylon.

Natural polymers

Natural polymers are polymers obtained from living things such as plants and animals.

Natural rubber is obtained from rubber trees as latex.The monomer for the natural rubber is

isoprene or 2-methylbuta-1, 3-diene.Each isoprene unit has two double bonds.Isoprene

molecules undergo addition polymerization to form poly (isoprene) or natural rubber.

Starch and cellulose are carbohydrates..They are formed by plants trough polymerisation of

simple sugar molecules called glucose.

Wool, meat and silk are made up of a variety of proteins.They are all naturally occurring

polymers that consist of amino acids as the monomers.

Synthetic polymers

Synthetic polymers are polymers made by man trough chemical reactions.

Monomers used for the manufacture of synthetic polymers are usually obtained from the

fractional distillation of petroleum.

Synthetic polymers are used to make plastics, fibres, resins and synthetic rubbers.

How are synthetic polymers made?

Synthetic polymers are prepared trough two types of polymerisation processes :

a) Addition polymerisation

b) Condensation poymerisation

Addition polymerisation involves monomers with double bonds between the carbon

atoms.For example, ethane, chloroethene, and styrene undergo addition polymerisationto

form poly (ethane) , polychloroethene and polystyrene respectively.

During addition polymerisation, the double bonds between pairs of carbon atoms break and

the carbon atoms of adjacent ethane molecules join together to form a molecule of poly

(ethene) or polyethethene.

addition polymerisation

Condensation polymerisation involves the joining up of other smaller and simple molecules

such as water.Examples of polymers produced by condensation polymerisation are nylon

and terylene.

Plastics

The raw materials used to make plastics are obtained from the products of cracking

petroleum fractions.They are normally alkene molecules and are made into plastics trough

addition polymerisation.

Plastics are the largest group of synthetic polymers with the following properties :

a) Can be easily moulded

b) Low density

c) Strong

d) Inert to chemicals

e) Insulator of heat and electricity

f) Can be coloured

The table shows some examples of commonly used plastics, how they are formed, their

properties and uses.

Name of polymer Equation for polymerisation Properties Uses

Polyethylene

(polyethene)

Durable, light,

impermeable, inert to

chemicals, easily

melted, insulator

Shopping bags,

plastic cups and

plates, toys

Polypropylene

(poypropene)

Durable, light,

impermeable, inert to

chemicals, easily

melted, insulator, can

be moulded and

coloured

Bottles, furniture,

battery casing,

pipes, toys

Polyvinyl chloride

(polychloroethene)

Low softening

temperature, durable,

elastic, can be

coloured

Pipes, pipe

fittings, wire and

cable casing,

raincoats,

footwear, bags

Polystyrene Heat insulator, light,

can be moulded,

impermeable

Disposable cups

and plates,

packaging

materials, toys,

heat insulators

Perspex Transparent, strong,

light

Replacements for

glass, lenses and

optical fibres

Teflon Durable, non-stick,

chemically inery,

strong, impermeable

Coatings for non-

stick pans,

electrical

insulators

Synthetic fibres

Synthetic fibres are long-chain polymers which are not easily stretched and have high

strength.

Polymers and polyesters are two groups of synthetic polymers used as fibres for making

textile.

Nylon is an example of polyamide polymers.

Terylene is an example of polyester polymers.

Nylon and terylene are produced trough condensation polymerisation.

Nylon is a general term given to the synthetic polymer made from two types of monomers,

diamine molecules and diacid molecules.

A diamine molecule has two amino groups, -NH2. A diacid molecule jas two

carboxyl groups, -COOH.

Diacid molecules and diamine molecules undergo condensation reaction to form

nylon and water.

H

For example, Nylon 6,6 is manufactured from polymerisation of hexane-1,6-dioic

acid, HO2C(CH2)4CO2H and hexane-1,6-diamine, H2N(CH2)6NH2.

H

Nylon 6,6 is a strong, tough, and waterproof polymer and can be easily made into

fibres.

Nylo is used to make toothbrushes, ropes, fishing lines, parachutes, carpets, textile,

threads and electrical isulators.

The monomers used to make terylene are diol molecules and diacid molecules.

A diol molecules has two hydroxyl groups, -OH. A diacid molecule has two

carboxyl groups, -COOH.

Diacid molecules and diol molecules undergo condensation reaction to form

polyester and water.

H

Terylene is manufactured from ethane-1,2-diol, HO(CH2)2OH and benzene-1,4-

dicarboxylic acid, HO2C(C6H4)CO2H.

H

Diol

Terylene is chemically inert, elastic and can be coloured and easily made into

fibres.

Terylene is suitable for making textile, stocking, parachutes and fishing nets.

Synthetic rubbers

Synthetic rubbers are polymers that are elastic and can regain their original length and

shape after being stretched and pressed.

The structure of the monomers of synthetic rubbers are similar to that of natural rubber.

Synthetic rubbers are produced by addition polymerisation.

Neoprene and styrene-butadiene rubber (SBR) are examp;es if synthetic rubbers.

Synthetic polymers in daily life

o Synthtic polymers have been widely used and have replaced many other building materials.

o About 57% of the total production of polymers are used in packaging and buiding

industries.

o The uses of a synthetic polymer are determined by its structure and properties.

o One of the great adveantages of synthetic polymers is that they can be made with special

properties to suit certain specifications

o Synthetic polymers have been widely used to replace metals, wood, clay, and natural

rubber.

o Synthetic polymers are very stable.Unlike metals, wood or paper, they do not rust, rot or

decay.This property of synthetic polymers make them very useful, but this also means that

they are very difficult to dispose of as they are not easily biogradable.

o Disposal of synthetic polymers has caused environmental pollution problems.

a. Synthetic polymers are not easily biodegradable, thus their waste will block or clog

up the drainage system, thereby causing flash flood.

b. Plastic containers which are not properly disposed of will become breeding ground

for mosquitoes which will cause the spread of diseases such as dengue.

c. The waste plastics pollute the lake and river, making the water not suitable for

aquatic organisms to live in.

d. The burnig of synthetic polymers will produces gases like carbon monoxide,carbon

dioxide, hydrogen chloride, sulphur dioxide and oxides of nitrogen.

These gases can cause the greenhouse effect and contribute to the acid rain

problem.The burning of PVC release choking fumes of poisonous hydrogen chloride gas

o The main source of raw materials for the manufacture of synthetic polymers is the

petrochemical feedstocks derived from petroleum, a non-renewable resource.

o There are a number of ways to solve the problems caused by the use of synthetic polymers.

a) Reuse

b) Recycle

c) Use biodegaradable synthetic polymers

d) Dispose of unwanted synthetic polymers in a proper manner.

o Reuse

Many plastic containers can be collected, cleaned and reuse for a certain number of times.Used

plastic materials can be converted into decorative items.

o Recycle

Many polymers can be recycled.Plastic drink bottles are collected, cleaned and then shredded

into chips.The chips are melted and then moulded to produce new items.

Recycling of plastics can only be carried out for a limited number of times because

remelting and remoulding graduaaly degrade the plastics.

The other problem faced in recycling are collection and separation of plastics.The plastics

must be separated into different types before sending them for recycling.

o Use biodegradable synthetic polymers

Scientists have developed biodegradable and photodegradable plastics.These plastics can be

decomposed by bacteria or sunlight.

Biopol (polyhydroxybutyrate) is an example of biodegradable plastic.

Conventional plastics can be made biodegradable by incorporating starch into their

structures.The starch can be digested by bacteria, leaving behind a flimsy plastic residue.

o Disposal of unwanted synthetic polymers in a proper manner

Burning plastics will convert the carbon and hydrogen in plastics to carbon dioxide and water

respectively.However, burning of certain plastics produces a variety of toxic gases.

Burying is cheaper option to dispose of plastic waste.However, most plastics are non-

biodegradable.

Pyrolysis is a procees of heating the plastics in the absence of air to break them down.

The needs for new materials

With our fast expanding modern technologies nowadiays, we need to produce new

materials with unusual combinations of properties.

These special, specified, diversified and varied propertiescan not be met by the

conventional metal alloys, ceramics and polymeric materials.

Continuous research and has been done in searching for new structural materials to fulfill

these needs.These building materials must have varied and diversified properties such as

light, strong stiff, abrasive, and resistant to heat, impact and corrosion.

In designing new building materials, scientists and engineers combine various

metals,ceramics and polymers to produce new generation of extraordinary materials.

What are composite materials?

A composite material is a structural material that is formed by combining two or more

different materials such as metals, alloys, glass, ceramics and polymers.

A composite material combines the advantages and strength of the chosen materials

without their weaknesses.

A composite material has properties that superior to those of the original components.It is

created for aspecific purpose and use.

Wood and bone are examples of natural composite materialsl.Wood consists of strong and

flexible cellulose fibres surrounded and held together by a stiffer material called

lignin.Bone is made up of living cells in a matrix of collagen fibres and calcium salts.

There are many types of composite materials being invented and used in our daily life

today.

Examples of common composite materials are reinforced concrete, superconductor, fibre

optic, fiberglass and photochromic glass.

Reinforced concrete

Concrete is a composite material which consists of a mixture of stones, chips and sand

bound together by cement.It is strong but brittle and weak in tensile strength.

Steel is strong in tensile strength

When concrete is reinforced with steel wire netting or steel rods, the resulting

combination is a very tough material with high tensile strength.This material is known

as the reinforced concrete.

Both steel and concrete have about the same coefficient of expansion.They make very

good composites and are essential for the construction of large structures like high-rise

buildings, bridges and oil platforms.

Reinforced concrete is relatively cheap and can be moulded into any shape.Thus, it

needs a very low building cost and very little maintenance.

Concrete can also be reinforced wth other materials like glass fibres, polymer fibres,

silicon carbide and aluminium oxide particles.

The fibre reinforced concrete has a greater strength than ordinary concrete and has

higher resistance to impact.It has been used in the construction of roads,aircrafts

runaways and rocket launching pads.

Concrete reinforced with polymer fibres is suitable to build buildings that are subject to

earthquakes.

Superconductors

Superconductors are capable of conducting electricity without any electrical resistance

when they are cooled to an extremely low temperature.

Most of the superconductors are alloys of metal compounds or ceramics of metal oxides.

Some superconductors are made from composite materials.These specific superconductors

are widely used today.

Superconductors are used in the bullet trains in Japan and medical magnetic-imaging

devices like magnetic resonance imaging (MRI).

Superconductors are also used in :

a) magnetic enrgy-storage systems

b) magnetically levitated train

c) generators

d) transformers

e) computer parts

f) very sensitive devices for measuring magnetic field, voltage or cirrent

Devices made by superconductors have low power dissipation, high speed operation and

high sensitivity.

Fibre optic

A fibre optic cable consists of a bundle of glass or plastic threads that are surrounded by a

glass cladding.

Fibre optic transmits data, voice and images in a digital format over long distances without

distortion and loss of signal, using light waves that pass trough the fibre.

It is used to replace copper wire in long distance telephone lines, in mobile phones, video

cameras and to link caomputres within local area networks.

It is also used in instruments for examining internal parts of the body or inspecting the

interiors of manufactured structural products.

Fibre optic is widely used because of its low material costs, high transmission capacity and

chemical stability.It is also less susceptible to interference

Main disadvantages of fibre optic cables are much more expensive to install, more fragile

and are difficult to split.

Fibreglass

Glass is hard , strong and has arelatively high density but is relatively brittle.

Plastic is elastic, flexible with low density but not that strong.

Fibreglass is produced when glass fibres are embedded in plastic resins to produce glass

fibre reinforced plastics.

Fibreglass has high tensile strength, can be easily coloured, moulded and shaped, inert to

chemicals and is low in density.It can be made into layers, yet very strong.

Fibreglass is used in the making of water storage tanjs, badminton rackets, helmets, small

boats, skis and car bodies.

Photochromic glass

A photochromic material is one that changes from transparent to coloured when it is

exposed to ultra violet light, and reverts to transparency when the light is dimmed or

blocked.

A photochromic glass can be produced by embedding photochromic substances like fine

silver chloride or silver halide crystals in glass or transparent polymers.

Whena photochromic glass is exposed to light, silver chloride is converted to silver and the

glass darkens.It becomes transparent again when siver is converted back to silver chloride

when the light dims.

Photochromic glass is used for making optical lenses, car windshields, smart energy

efficient windows in buildings, information display panels, lenses in cameras, optical

switches and light detector devices.

Photochromic glass helps to

a) protect our eyes from harmful ultraviolet rays and glare from the sun

b) control amount of light passes trough it automatically

c) reduce refraction of light