Chemistry notes, chapter 8
Manufactured Substances in Industry
Textbook Chemistry Form 4 KSSM (DLP), pages 252–274
What this chapter covers
- Describe alloys with examples, compare an alloy with its pure metal and justify the uses of alloys (8.1)
- Describe the types of glass, their composition, properties and uses (8.2)
- Describe ceramics, their composition, properties and uses in daily life (8.3)
- State the meaning and properties of composite materials and compare them with their constituent materials (8.4)
8.1Alloy and its importance
An alloy is a mixture of two or more elements where the main element is a metal. A pure metal is made of only one type of atom.
Why an alloy is harder than its pure metal
In a pure metal the atoms are the same size and sit in orderly layers. When a force is applied the layers slide easily over each other, so a pure metal is ductile (pulled into wires) and malleable (shape changed easily).
In an alloy, foreign atoms of a different size disrupt the orderly arrangement. The layers cannot slide easily, so the alloy is harder and stronger. Alloying also helps to prevent corrosion.
Figures: textbook Figures 8.5 and 8.7, p. 257.
Do not say the alloy has "no spaces" or that its atoms are "all the same size". The point is that atoms of different sizes disturb the layers so they cannot slide.
Experiment 8.1: alloy and pure metal
- Corrosion: stainless steel and iron plates in distilled water for a week. Iron rusts, stainless steel does not (manipulated: type of plate; responding: corrosion).
- Hardness: a 1 kg weight dropped from a fixed height onto a steel ball bearing on a bronze block, then on a copper block. The smaller the dent, the harder the block. The bronze dent is smaller, so bronze (alloy) is harder than copper (pure metal).
Composition, properties and uses
| Alloy | Composition | Properties | Uses |
|---|---|---|---|
| Duralumin | 93% Al, 3% Cu, 3% Mg, 1% Mn | Stronger than Al, low density, does not rust | Aeroplane body, electric cables, racing bicycles |
| Bronze | 90% Cu, 10% Sn | Stronger than Cu, does not rust, shiny | Medals, monuments, trophies |
| Brass | 70% Cu, 30% Zn | Stronger than Cu, does not rust, shiny | Musical instruments, doorknobs, keys |
| Steel | 98% Fe, 0.2–2% C | Stronger, harder, malleable | Buildings, railway tracks, car body |
| Stainless steel | 73% Fe, 18% Cr, 8% Ni, 1% C | Stronger than Fe, resists corrosion | Cutlery, sinks, surgical instruments |
| Pewter | 95% Sn, 3.5% Sb, 1.5% Cu | Stronger than Sn, does not rust, shiny | Ornaments, trophies, souvenirs |
Percentages may vary. A superconductor has no electrical resistance at very low temperature and is used to make the magnets of Maglev trains.
Worked example18 K gold is 18 parts gold and 6 parts other metal in 24 parts. For 24 g: gold = 18 g (75%), copper = 6 g (25%). Copper makes the gold harder and stronger.
Why is stainless steel used to make sinks?
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It contains chromium and nickel, so it resists corrosion and is strong and shiny.
8.2Composition of glass and its uses
All glass is based on silica (silicon dioxide, SiO2). Basic properties: hard but brittle, transparent, chemically inert, waterproof, heat and electrical insulator.
| Type | Made from | Special property | Uses |
|---|---|---|---|
| Fused silica glass | SiO2 only | Very high melting point (about 1800 °C), hardly expands or contracts | Telescope lenses |
| Soda-lime glass | SiO2, Na2CO3, CaCO3 | Low melting point (about 1000 °C), easily moulded, cracks with sudden temperature change | Bottles, jugs |
| Borosilicate glass | SiO2, Na2CO3, CaCO3, B2O3, Al2O3 | Heat resistant, low expansion coefficient | Beakers, flasks |
| Lead crystal glass | SiO2, Na2CO3, PbO | Soft, dense, high refractive index | Prisms |
Soda lowers the melting point of silica. Boron oxide gives heat resistance. Lead replaces calcium to make the glass denser.
Why is borosilicate glass used for laboratory glassware?
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It has a low expansion coefficient, so it does not crack when heated or cooled quickly.
8.3Composition of ceramics and its uses
A ceramic is an inorganic, non-metallic solid made by shaping and then hardening at high temperature. Most are compounds of metals, non-metals or metalloids.
Atoms in ceramics are held by strong ionic and covalent bonds in an indefinite arrangement and cannot slide, so ceramics have a high melting point, are hard and strong, chemically inert and insulators, but are brittle and weak when stretched.
| Type | Made from | Examples and uses |
|---|---|---|
| Traditional | Clay such as kaolin, Al2O3·2SiO2·2H2O | Bricks, pottery, crockery |
| Advanced | Oxides, carbides and nitrides | Silicon carbide in cutting and brake discs; tungsten carbide rings; zirconia dental implants; alumina knee bones; insulators in power stations; jet engine parts |
A ceramic is not a metal. It is an inorganic, non-metallic substance, and its brittleness is the reason it is not used where it must bend or stretch.
Can silicon carbide be used to make a drinking glass? Explain.
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No. It is very hard, brittle and not transparent like glass; it is made for hard, high-temperature uses such as cutting discs and brake discs, not for containers for drinks.
8.4Composite materials and their importance
A composite material combines two or more non-homogeneous substances: a matrix that surrounds and binds a strengthening substance. The result has better properties than the original components.
| Composite | Matrix | Strengthening | Better property and use |
|---|---|---|---|
| Reinforced concrete | Concrete | Steel bars or wire mesh | Withstands compression and stretching; bridges, dams, buildings |
| Fibre glass | Plastic | Glass fibres | Strong, light, durable; helmets, car bumpers, circuit boards |
| Optical fibre | Glass or plastic cladding and jacket | Silica glass core | Carries data as light by total internal reflection; replaces copper wire |
| Photochromic glass | Glass | AgCl and CuCl | Darkens in sunlight (silver atoms form), clears in dim light; car windows, lenses |
| Superconductor (YBCO) | Yttrium, barium, copper oxides | No resistance at very low temperature; MRI, NMR, particle accelerator magnets | |
Composite and its original components (8.4.3)
| Composite | Original components | Composite is better because |
|---|---|---|
| Reinforced concrete | Concrete: high compression strength, low stretching strength. Steel: high stretching strength, corrodes easily | High compression and stretching strength, resistant to corrosion |
| Fibre glass | Plastic: weak, soft, easily burned. Glass fibre: high stretching strength | High stretching strength, durable, resistant to corrosion, insulator |
| Optical fibre | Glass fibre: hard but low compression strength. Plastic: flexible | High compression strength and flexible |
| Photochromic glass | Glass: does not absorb UV rays. Silver chloride: absorbs UV rays, sensitive to light | Transparent, absorbs UV rays, and the absorption depends on light intensity |
| Superconductor (YBCO) | Its starting compounds have high electrical resistance at room temperature | No electrical resistance at very low temperature |
For each composite, name what each part contributes. Concrete resists compression; steel resists stretching and gives corrosion resistance.
State the matrix and strengthening substance in fibre glass.
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Matrix: plastic. Strengthening substance: glass fibres.