2 4 Alloys

IV. Alloys

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Topic

IV.

Alloys

Reference Reading

Integrated Chemistry Today, L.H.M. Chung, Book 1 pg. 236 – 242, 246

Objectives

2.4 – recognise that alloys are formed by the introduction of other elements into metals – recognise the major constituents of the following alloys (a) brass: copper and zinc (b) steel: iron, carbon and other metals (c) solder: tin and lead (d) coinage metal: e.g. nickel and copper (e) gold of different carats: gold, silver and copper in different proportions – relate the use of the above alloys to their properties as compared with the pure metals (from given information)

Notes

IV. Alloys Alloy - Homogenous mixture of metal. When a liquid metal is mixed with another liquid metal (or other substances), an alloy (solid metal solution) is formed. Alloy is only a mixture but not a compound, because it does not have a fixed composition. e.g. Different stainless steel has different amount of iron, chromium, nickel, carbon and other ingredients. e.g. In mild steel, it contains iron and carbon only. A. Structure of alloy Alloy is usually harder than pure metal. In pure metal, atoms are regularly arranged. When a force is applied, the atoms may slide over each other.

Addition of exotic atoms causes a distortion of the regular arrangement. This prevents the smooth sliding of atom layers over each other. Thus, hardens the structure.

IV. Alloys

Page 2 B. Constitutions of different alloys Brass : Copper and zinc Characteristic : Hard, resistant to corrosion Use : 1. Water pipe – resistant to corrosion 2. Metal screw – hard Steel : Iron, carbon and other metals Characteristic : Strong, cheap and more resistant to corrosion Use : 1. Major structure e.g. bridge, building Solder : Tin and lead Characteristic : Low melting point Use : 1. Soldering metal Coinage metal : e.g. Nickel and copper Characteristic : High abrasion resistance and appropriate hardness Use : 1. Coin - Strong but soft enough for the pressing the figure on it. Carat gold : Gold, silver and copper Characteristic : Harder and cheap than pure gold. Use : 1. Jewelry - more durable than pure gold and easier to shape.

C. Use of alloy Alloy Mild Steel

Composition iron, carbon

Properties strong but malleable

Stainless Steel

iron, chromium, nickel

hard, very corrosion resistant

Brass

copper, zinc

Duralumin

aluminium, copper, magnesium, manganese lead, tin copper, tin

golden appearance, malleable, resist corrosion, harder than the pure metals light, strong and more corrosion resistant than aluminium very low melting golden appearance, resist corrosion and wearing soft enough to be minted, hard enough not to wear away easily. golden appearance, hard

Solder Bronze Coinage metal Carat gold

Glossary

1. copper, tin, zinc 2. nickel, copper gold, silver, copper

Uses making structures and commodities like tools, magnets for structural uses like car bodies, bearings, cutlery making ornaments, buttons, screws and electrical fittings making aircraft and spacecraft for welding making ornament 1. copper, tin, zinc (copper coins) 2. nickel, copper (silver coins) making ornaments, jewellery

alloy homogenous mixture imperfections exotic atoms constitutions brass steel solder coinage metal carat gold mild steel stainless steel duralumin cutlery ornament

IV. Alloys

Past Paper Questions

Page 3 95 I 6 b iii 96 I 8 b i ii 97 I 1 b 99 I 7 a ii iii

97 I 8 b iv

95 I 6 b iii 6b The table below gives some information about five metals. Metal Abundance in the Price per kg ($) Relative resistance of corrosion Relative strength of metal earth’s crust (%) (1 = least, 4 = most resistant) (1 = lowest, 3 = highest) Al 8.1 170 3 1 Cu 0.005 140 3 3 Au 0.0000004 1100000 4 2 Fe 5.0 20 1 3 Zn 0.007 160 2 2 iii (1) Aluminium does not corrode easily. Why ? 3 (2) Aluminium is a principal material for making aircraft but its strength is relatively low. Suggest how the strength of aluminium can be improved to make it suitable for making aircraft. (1) Al reacts with oxygen in air to form a (thin) layer of oxide which is impervious (not permeable) to oxygen / water and prevents the metal from further corrosion. 2 marks (2) Alloying with other metals (e.g. Cu / Mn / Mg...) 1 mark (Do not accept mix with other metals. Do not accept K / Na / Ca / Hg) C (1) Most candidates did not mention that the oxide layer is impervious to oxygen or water. (2) Some candidates used the term 'mixture of metals' instead of 'alloy'. A number of candidates wrongly thought that anodization can improve the strength of aluminium to make it suitable for making aircraft. 96 I 8 b i ii 8b The diagram below shows a ten dollar coin which is made of two alloys, X and Y.

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(1) What is an alloy ? (2) State TWO properties of an alloy that make it suitable as a coinage metal. (1) An alloy is a (solid) mixture of two or more elements of which at least one is a metal / a (solid) mixture of metals 1 mark (2) Any TWO of the following: It should be soft enough /easy to be minted / stamped (moulded) It should be corrosion resistant / chemically unreactive / lone lasting / durable It should be hard enough to resist it from wearing away 2 marks Most candidates did not give a precise meaning of the term 'alloy'. Some candidates even wrote that an alloy is a compound formed by two or more metallic elements. When compared with using only one alloy, using two alloys to make the ten dollar coin has certain advantages and disadvantages. Give ONE advantage and ONE disadvantage of using two alloys to make the coin. Advantage: more attractive (beautiful) / prevent forgery / can easily be distinguished from other coins 1 mark Disadvantage: difficult to make / more expensive / possibility of X detaching from the coin 1 mark (Accept any reasonable answer)

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IV. Alloys Page 4 97 I 1 b 1 For each of the tasks listed in the table below, decide which substance on the right is the best to use to accomplish the task. Explain your answer in each case. For each of the following, award 1 mark for the correct choice and 2 marks for the explanation; (must involve a sense of comparison); disregard the explanation for a wrong choice. i.e. zero mark is given to a wrong choice. Task Substance 1b To make the body of an aircraft aluminium, duralumin, 3 stainless steel duralumin 1 mark Stainless steel has the highest density / is too heavy for making aircraft 1 mark or, duralumin is lighter than stainless steel / has a lower density than stainless steel. (1 mark) duralumin is more corrosion resistant / (mechanically) stronger than aluminium. 1 mark C Most candidates chose the correct substances to accomplish the tasks in (b), but they failed to give sound arguments to explain their choices. In making their arguments many candidates cited properties of the substances chosen, e.g. duralumin is light and strong. They did not compare these properties with those of the other substances. This indicated that these candidates were weak in presenting arguments based on known facts. 97 I 8 b iv 8b Read the following paragraph concerning chromium and answer the questions that follow: The Greek word "chroma" means colour. Many chromium-containing compounds and chromium containing gemstones are beautifully coloured. The oxidation number of chromium in its compounds can be +2, +3 and +6. iv Suggest TWO ways in which chromium can be used to prevent the corrosion of iron. making stainless steel / alloy 1 mark chromium-plating 1 mark C In part (iv), only a few candidates correctly pointed out that making stainless steel is one of the ways in which chromium can be used to prevent the corrosion of iron. 99 I 7 a ii iii 7a The table below lists the key components of three alloys. Alloy Key components bronze copper, tin steel iron, carbon solder tin, metal X ii Steel is an alloy of iron and carbon. Explain, in terms of structure, why the addition of carbon to iron can enhance the strength of iron. iii Solder is an alloy of tin and metal X. (1) What is X ? (2) Explain why solder, but not tin or X, is used for joining electrical wires. 96 36 B

98 40 A

36 Which of the following metals is/are present in solder ? (1) copper (2) lead (3) iron A. (1) only B. (2) only C. (l) and (3) only D. (2) and (3) only

40 Brass is commonly used for making ornaments because (1) it has an attractive appearance. (2) it can resist corrosion. (3) it is softer than copper. Which of the following combinations is correct ? A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3)

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