Materials

20.1Materials and their properties

Look around a classroom: the desk is wood, the window is glass, the fan is metal and plastic, and the floor is concrete or ceramic tiles. A material is the substance or mixture that an object is made of. Everything around us is made from materials, and each material has its own set of properties. Materials science studies these properties and how to change them so that the right material can be chosen for each job.

Material

A substance or mixture of substances that makes up an object, for example plastics, metals, fabric and glass.

Property of a material

A characteristic, such as strength, flexibility, conductivity or melting point, that decides how the material can be used.

Properties of materials (20.1.1)

The properties that engineers look at most are listed below. They depend on two things: the chemical composition of the material (which atoms it contains) and its internal structure (how the atoms are arranged and bonded).

Changing properties by processing

Processing methods such as mixing, heating and cooling change the internal structure of a material and so change its properties (Fig 20.1 of the book shows wood being processed into paper pulp in stages).

• Mixing: when two substances are mixed, a new material with its own properties forms. Iron mixed with a little carbon gives steel, which is much harder than iron.
• Heating: can make some materials more flexible or change their conductivity.
• Cooling: can make a material harder or more brittle; rapid cooling of hot steel (quenching) makes it very hard.

Choosing a material

The final choice also depends on cost, colour, texture and appearance. For example, aerospace engineering needs a material that is both strong and light, so aluminium alloys and composites are used instead of steel. Thus the combination of properties, processing and cost decides which material is used for a desired structure.

20.1.2Alloys and metal compoundsOften examined

Pure metals are often too soft, too weak or too easily corroded for use. Iron, the most used metal in the world, is fairly soft in its pure form, but adding a very small amount of carbon makes it hard. Such mixtures of metals are called alloys. By alloying, the important properties of a metal can be improved.

Alloy

A homogeneous mixture formed by mixing a metal with one or more other metals or non-metals (such as carbon) in a certain proportion.

Base metal

The main metal in an alloy, for example iron in steel or copper in brass.

Most metals occur in nature as compounds (oxides, sulphides, carbonates) in their ores. The metal is produced (smelted) from these compounds by the methods of section 20.2 and is then mixed with other elements to make an alloy. Properties such as shape and mass may differ between objects made of the same material, so density (mass per unit volume) is used to compare different materials.

Common alloys

Properties of alloys (20.1.3)

1. Stronger than the metals from which they are made.
2. Harder than the constituent metals.
3. More resistant to corrosion, for example stainless steel and bronze.
4. Lower melting points than the constituent metals, for example solder melts below both tin and lead.
5. Lower electrical conductivity than pure metals. Copper is an excellent conductor, but brass and bronze are poorer conductors.
6. They are characteristically shiny, and most are malleable (can be hammered into sheets; gold can be beaten into an extremely thin sheet) and ductile (can be drawn into wires).

Why alloys are harder and stronger

In a pure metal all the atoms are the same size and are arranged in regular layers. When a force is applied, the layers slide over each other easily, so the metal is soft and bends. In an alloy, atoms of a different size are scattered among the host atoms. They distort the layers, so the layers cannot slide easily, and the alloy is harder and stronger.

20.1.2Plastics and ceramicsOften examined

Plastics

All plastics are made from elements such as hydrogen, carbon, oxygen, nitrogen and other non-metals, joined into long-chain molecules called polymers (Unit 13). The word poly means many. Plastics and rubbers include polyethylene, nylon, polystyrene and PVC. Most of the hydrocarbons used to make plastics come from oil and natural gas. During processing, additives (plasticisers, colours, fillers, stabilisers) are mixed in to give the plastics special characteristics.

The monomers of three common plastics are shown below; each has a $\mathrm{C}=\mathrm{C}$ bond that opens during addition polymerisation.

Properties of plastics

1. Low density, so they are light in weight.
2. Chemically inert (unreactive) and chemically stable.
3. Good electrical insulators and poor conductors of heat (low thermal conductivity).
4. Easily moulded into different shapes and sizes.
5. Low cost and easy to manufacture.
6. Used in electronic devices and household gadgets as insulators.

Ceramics

Ceramics are hard, heat-resistant compounds of metallic and non-metallic elements (mostly oxides and silicates), made by heating clay or other minerals to a high temperature. They include cement, glass and clay minerals such as porcelain. Tiles are made from silica, glass and porcelain. Ceramics give excellent thermal insulation. Some ceramics are magnetic: a common ceramic magnet is made from strontium and iron oxides.

Properties of ceramics

1. Very hard.
2. Brittle: they crack instead of bending.
3. Stiffness and strength comparable to metals (especially when compressed).
4. Poor conductors of heat and electricity.
5. Can be transparent, translucent or opaque.
6. Very high melting points and chemically resistant.

Uses of ceramics

• floor and wall tiles, bricks, pottery, sanitary ware (porcelain)
• glass for windows, bottles and lenses
• insulators on electric poles, spark plugs
• furnace linings and heat shields
• ceramic magnets in loudspeakers and motors

20.1.2Concrete and steel

Concrete

Concrete is one of the most commonly used building materials. It is made from two main ingredients:

1. A paste of water and cement (the binding agent).
2. Filler materials (aggregates) such as sand, crushed stone and gravel.

Portland cement ($10$ to $15\%$) and water ($15$ to $20\%$) form a paste, which is mixed with the aggregates. The cement reacts with water and hardens over time, binding the aggregates into a solid, strong mass.

Properties of concrete

• Affordable and durable.
• Can be moulded into nearly any shape before it sets.
• Very strong in compression (squeezing) but weak in tension (pulling), so steel bars are placed inside it to make reinforced concrete.

Steel

Steel is an alloy made mainly of iron with a small amount of carbon and sometimes other elements. The carbon makes steel strong and durable. Adding chromium, nickel, molybdenum or silicon gives different types of steel with different properties (for example stainless steel).

Properties of steel

1. High strength, so it can carry structural loads.
2. Highly durable and rigid.
3. Versatile: rolled, forged or cast into many shapes.
4. Most steels are easily weldable.
5. Lower thermal and electrical conductivity than many other metals.
6. Can be completely recycled.
7. Can be alloyed with chromium, nickel and molybdenum to resist corrosion.

20.1Comparing classes of materialsOften examined

The table below brings together every class of material in this unit. Use it to answer 'compare' questions and to choose a material for a structure.

Matching a material to a structure

20.2Extracting metals from oresOften examined

A mineral is a naturally occurring compound of a metal. An ore is a mineral from which the metal can be extracted profitably. The ore is mixed with earthy impurities called gangue. Extraction (metallurgy) is the series of steps that turns the ore into pure metal.

Ore

A naturally occurring mineral from which a metal can be extracted economically, for example haematite $\mathrm{Fe}{}_2\mathrm{O}{}_3$.

Gangue

The unwanted earthy and rocky impurities (sand, clay) mixed with an ore.

Metallurgy

The process of extracting a pure metal from its ore.

How reactivity affects extraction

The reactivity of a metal decides how it is extracted. A more reactive metal holds on to oxygen or sulphur more strongly, so a stronger method is needed to free it.

Overview of the steps

1. Crushing, grinding and concentration

Crushing and grinding: the ore is broken into smaller pieces and then ground to a fine powder, so that the ore particles separate from the gangue.

Concentration: the gangue is removed to increase the percentage of metal compound in the ore. Common methods:

2. Roasting

The concentrated ore is heated in air below its melting point so that it is converted into its oxide. Sulphur escapes as $\mathrm{SO}{}_2$. Examples: zinc blende, cinnabar and copper pyrite.

3. Smelting

Smelting is reducing the metal ions to free metal by heating the ore with a reducing agent. The most common reducing agents are coke (carbon), carbon monoxide and hydrogen:

Smelting of copper (two steps)

(i) Matte formation. The roasted copper ore is heated with coke and sand ($\mathrm{SiO}{}_2$) at about $1100{}^{\circ }\mathrm{C}$. The materials melt and separate into two layers. The bottom layer, a mixture of $\mathrm{Cu}{}_2\mathrm{S}$ and $\mathrm{FeS}$, is called matte. The upper layer is a silicate slag formed from $\mathrm{FeO}$ and sand:

(ii) Bessemerisation. Air is blown through the molten matte in a Bessemer converter. Any remaining $\mathrm{FeS}$ is oxidised and removed as slag ($\mathrm{FeSiO}{}_3$). Then part of the cuprous sulphide is oxidised to cuprous oxide, which reacts with the remaining cuprous sulphide to give metallic copper:

The product, blister copper, is about $97$ to $99\%$ pure. It has a blistered surface because bubbles of $\mathrm{SO}{}_2$ escape as it solidifies. The Bessemer process is also used to convert pig (cast) iron into steel by burning off the excess carbon.

4. Refining (purification)

The metal from smelting still contains impurities, so it is refined.

(i) Electrolytic refining

An electrolytic cell is used in which the impure metal is the anode and a thin sheet of pure metal is the cathode. For copper, the electrolyte is acidified copper sulphate solution. When current passes, copper at the anode dissolves as $\mathrm{Cu}{}^{2+}$ ions; these ions move to the cathode, gain electrons and deposit as pure copper. Less reactive impurities such as silver and gold do not dissolve and fall below the anode as anode mud; more reactive impurities (iron, zinc) stay in solution.

(ii) Distillation

Metals with low boiling points are refined by distillation: the impure metal is heated, the metal vaporises and is condensed, leaving non-volatile impurities behind. Mercury and zinc are purified in this way.

20.2Alloying, shaping and heat treatmentOften examined

After refining, the pure metal is usually made into an alloy and then shaped and heat treated to give it the required properties.

5. Alloying

Alloying is combining a metal with one or more other elements to improve its properties, such as strength, ductility, corrosion resistance and hardness. Methods:

• Melting and mixing: the base metal is melted, the alloying elements are added and the mixture is stirred well for uniform distribution. Steel is made by adding carbon and elements such as chromium, nickel and manganese to molten iron.
• Powder metallurgy: fine metal powders are mixed, pressed into the desired shape and heated so that the particles bond. Useful for alloys with special properties.
• Mechanical alloying: a mixture of powders is repeatedly fractured and welded together (in a ball mill) to make advanced alloys with unique properties.

6. Shaping

7. Heat treatment

Importance of alloy making (20.2.1)

1. Alloying increases the hardness of a metal.
2. Pure metals often have high melting points; alloying lowers the melting point range, which makes melting and casting easier.
3. Alloys resist corrosion and are not easily affected by chemicals or weather; for example bronze statues are not affected by sun and rain, and stainless steel does not rust.
4. Alloying changes the colour of the metal (brass is golden, German silver is silvery).
5. Alloys give better castings: many pure metals shrink (contract) on solidifying and leave cavities, while suitable alloys fill the mould well and give sharp castings.

20.3.1What is catalysisOften examined

Some reactions, such as the decomposition of hydrogen peroxide, are very slow on their own. Adding a pinch of manganese dioxide makes oxygen bubble out at once, yet the black powder is still there, unchanged, at the end. A substance that behaves like this is a catalyst.

Catalyst

A substance that increases the rate of a chemical reaction without being used up; it is chemically unchanged at the end and can be used again and again.

Catalysis

The process in which the rate of a chemical reaction is altered by a catalyst.

Activation energy, $E_a$

The minimum energy that colliding reactant particles must have to react (unit $\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$).

Promoter

A substance that makes a catalyst more effective, for example molybdenum with iron in the Haber process.

Inhibitor (catalytic poison)

A substance that makes a catalyst less effective, for example arsenic compounds poison platinum.

Characteristics of catalysts

1. A catalyst is not used up: its mass and chemical composition are the same at the end (its physical form may change, for example lumps become powder).
2. A small amount of catalyst is usually enough.
3. The catalyst often forms a temporary, unstable intermediate with a reactant and is then released, so it works in a cycle.
4. It lowers the activation energy; it does not change the overall energy change ($\Delta H$) of the reaction.
5. It speeds up the forward and backward reactions equally, so equilibrium is reached sooner but its position does not change.
6. A catalyst is often specific: one catalyst works for one reaction or a type of reaction.
7. Promoters increase and inhibitors (poisons) decrease its activity; the amount and surface area of catalyst affect the rate.
8. Catalysts can be stopped, weakened or destroyed by other processes (poisoning, high temperature).

Catalysed reactions have a lower activation energy, so at the same temperature and concentration a larger fraction of collisions have enough energy to react, and the reaction goes faster. The rate depends on how often reactants meet in the slowest step, which usually involves the catalyst.

Types of catalysis

20.3.2Mechanism of catalysisOften examined

A catalyst increases the rate of reaction by providing an alternative pathway (route) with a lower activation energy. It does this by temporarily bonding with the reactants, bringing them together in the right position and weakening their bonds.

If the activation energy is lowered from E_a to E_a', many more reactant particles have enough energy to cross the energy barrier, so more collisions are successful each second. The Arrhenius equation shows why a small drop in E_a has a big effect:

Here k is the rate constant, A the Arrhenius constant, E_a the activation energy, R=8.314\,\mathrm{J\,K^{-1}\,mol^{-1}} and T the temperature in kelvin. E_a is in the exponent, so lowering it increases k exponentially.

Why transition metals are good catalysts

• They have incompletely filled d-orbitals, so they can both accept electrons from and donate electrons to reactant molecules. This lets them form temporary bonds on their surface (adsorption).
• They show variable oxidation states, so they can be oxidised and reduced back in a cycle, for example \ce{Fe^3+} and \ce{Fe^2+}.

Intermediate compound (homogeneous) mechanism

The catalyst \ce{C} reacts with one reactant to form an unstable intermediate, which then reacts with the other reactant to give the product and set the catalyst free:

Each step has a lower activation energy than the direct reaction \ce{A + B -> AB}. Example: nitrogen monoxide catalyses the oxidation of \ce{SO2}:

Adsorption (surface) mechanism of heterogeneous catalysis

Industrial and biological examples

20.4Challenges of recycling and toxicity of materialsOften examined

Recycling is the process of collecting and processing materials that would otherwise be thrown away as waste and converting them into new products. It saves raw materials, energy and landfill space. Recycling one glass bottle saves enough energy to light a 100\,\mathrm{W} bulb for four hours, and making aluminium from scrap uses only about 5\,\% of the energy needed to make it from bauxite.

The recycling problem involves challenges in collecting, sorting, managing and processing recyclable materials. These arise from poor waste management systems, improper waste disposal, lack of education and awareness, and difficulty in separating recyclable from non-recyclable waste. Pollution occurs when non-recyclable materials mix with recyclables. If recycling costs more than its benefits, it may not be worth doing. Items such as chip bags and juice pouches are hard to recycle because they are made of several layers of different materials.

Main challenges

Thermoplastics and thermosetting plastics

Whether a plastic can be recycled depends on its type, so recyclers must know which type they have.

Toxicity of materials

The toxicity of recycled materials varies with the type of material and the recycling process. Safe recycling practice and thorough testing of recycled materials are needed to reduce harm.

20.5X-ray crystallographyOften examined

We cannot see atoms with a microscope that uses light, because the wavelength of light (about 500\,\mathrm{nm}) is thousands of times larger than an atom. X-rays have wavelengths of about 0.1\,\mathrm{nm}, which is about the same as the distance between atoms in a crystal. So a crystal acts like a three-dimensional diffraction grating for X-rays.

X-ray crystallography

A technique that uses the diffraction of X-rays by a crystal to find its detailed structure: the arrangement of atoms and molecules, bond lengths and bond angles, and any irregularities in the crystal.

It is used on salts, minerals, metals, semiconductors and biological compounds such as proteins, DNA and vitamins, and so is valuable in biology, chemistry, geology and materials science.

How it works

1. A pure single crystal of the substance is grown and mounted in the X-ray beam.
2. A narrow beam of X-rays of one wavelength strikes the crystal.
3. The electrons of the atoms scatter the X-rays. Scattered waves from different layers of atoms interfere; in certain directions they reinforce (constructive interference) and give strong diffracted beams.
4. The crystal is rotated, and a detector records the angles and intensities of thousands of diffraction spots.
5. A computer converts this pattern into an electron density map, from which the positions of atoms, bond lengths and bond angles are found.

Bragg's law

20.5.1Applications of X-ray crystallographyOften examined

X-ray crystallography has been used to study many different molecules and was vital in several famous discoveries. The first structures solved were simple crystals such as sodium chloride and quartz. A major achievement was the double-helix structure of DNA (Franklin, Watson and Crick, 1953), based on Rosalind Franklin's X-ray photographs. Dorothy Hodgkin used it to find the structures of penicillin, **vitamin \mathrm{B_{12}} and insulin**.

Formula sheet

Key terms

Material

A substance or mixture that makes up an object.

Alloy

A homogeneous mixture of a metal with other metals or non-metals in a certain proportion, made to improve the metal's properties.

Malleable

Able to be hammered or pressed into thin sheets without breaking.

Ductile

Able to be drawn into wires.

Brittle

Breaking suddenly without bending, like glass or porcelain.

Plastic

A material made of long-chain synthetic polymers, usually from oil or natural gas, that can be moulded.

Ceramic

A hard, brittle, heat-resistant material made of compounds of metals and non-metals, such as porcelain, glass and cement.

Concrete

A building material made from cement, water and aggregates (sand, gravel, crushed stone) that hardens over time.

Steel

An alloy of iron with a small amount of carbon and sometimes other elements.

Composite

A material made of two or more materials that stay distinct but work together, such as reinforced concrete.

Ore

A mineral from which a metal can be extracted profitably.

Gangue

Earthy and rocky impurities mixed with an ore.

Concentration of ore

Removal of gangue from a powdered ore by froth flotation, magnetic separation or gravity separation.

Roasting

Heating a concentrated ore in air below its melting point to convert it into an oxide.

Smelting

Reducing metal ions in an ore to the free metal by heating with a reducing agent such as coke, CO or hydrogen.

Matte

The molten mixture of \ce{Cu2S} and \ce{FeS} formed in the smelting of copper.

Slag

The fusible waste, such as \ce{FeSiO3}, formed when impurities combine with a flux like sand.

Bessemerisation

Blowing air through molten matte (or pig iron) in a Bessemer converter to remove impurities by oxidation.

Blister copper

Copper of 97 to 99\,\% purity with blisters caused by escaping \ce{SO2}.

Refining

Purification of the impure metal obtained from smelting, for example by electrolysis or distillation.

Anode mud

The insoluble impurities, such as silver and gold, that collect below the anode in electrolytic refining.

Annealing

Heating a metal and cooling it slowly to remove internal stresses and improve ductility.

Quenching

Heating a metal and cooling it rapidly in water or oil to make it harder.

Tempering

Reheating quenched metal to a lower temperature and cooling it, to balance hardness and toughness.

Powder metallurgy

Making an alloy by pressing mixed metal powders into shape and heating them.

Catalyst

A substance that increases the rate of a reaction and remains chemically unchanged at the end.

Activation energy

The minimum energy colliding particles need in order to react.

Promoter

A substance that increases the activity of a catalyst.

Inhibitor

A substance that decreases the activity of a catalyst (a catalytic poison).

Homogeneous catalysis

Catalysis in which the catalyst and reactants are in the same phase.

Heterogeneous catalysis

Catalysis in which the catalyst is in a different phase from the reactants, usually a solid surface.

Adsorption

The holding of molecules on the surface of a solid.

Recycling

Collecting and processing waste materials and converting them into new products.

Thermoplastic

A plastic that softens on heating and can be remoulded, so it can be recycled.

Thermosetting plastic

A cross-linked plastic that does not soften on reheating, so it is hard to recycle.

X-ray crystallography

A method that uses diffraction of X-rays by a crystal to find the arrangement of its atoms and molecules.

Bragg's law

n\lambda = 2d\sin\theta, the condition for strong reflection of X-rays from planes of atoms.

FBISE 2026 questions

Questions from the FBISE HSSC-II 2026 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

Exam focus

1. Describe three properties of alloys that make them important. (short)
   Asked in 2026 Set D Q.2 (xiv); give each property with a reason or example.
   Go to answer
2. Explain the extraction of a metal: concentration, roasting, smelting (copper matte and Bessemerisation) and refining, with equations. (long)
   Long question i and short questions iv, vii and viii; the longest process in the unit.
   Go to answer
3. Mechanism of catalysis with an energy profile diagram and examples. (long)
   Long question ii, short question v and MCQ ii; catalysis was also asked in 2026 Set B Q.2 (xiv) with a graph.
   Go to answer
4. Principle (Bragg's law) and applications of X-ray crystallography. (long)
   Long question iv, short question ix and MCQ iii.
   Go to answer
5. Environmental and economic challenges of recycling; toxic materials. (long)
   Long question iii and short question ii.
   Go to answer
6. Compare plastics and ceramics; properties and uses of ceramics. (short)
   Short questions i and iii, MCQs iv, viii and x.
   Go to answer
7. Give examples of alloys with their components and uses. (short)
   Short question vi; learn steel, stainless steel, brass and bronze.
   Go to answer
8. Annealing, quenching and tempering. (mcq)
   MCQ v; easy marks if the cooling rates are remembered.
   Go to answer
9. Methods of extraction according to the reactivity series. (short)
   Fig 20.7 of the book; links extraction to reactivity.
   Go to answer