Chemistry - Alloys (AQA Syllabus A)

In GCSE Science one topic studied is building materials. This is the fifth of seven quizzes on that subject and it looks in particular at alloys of metals, their properties and how they are used.

Metals are one of the materials we see all around us in everyday life, but did you realise that they are nearly all in the form of alloys? An alloy is a substance composed of two or more metals, or of a metal or metals with a nonmetal. Alloys are used to alter the properties of a metal, for example, magnesium. This is a light, soft and highly reactive metal. When alloyed with aluminium it is light, strong and corrosion resistant and can be used in the making of aircraft and cars.

Alloys are not a new discovery. Humans have been using metals since the end of the stone age - first during the bronze age and then during the iron age. During the bronze age, only the low reactivity metals like gold, silver, copper and tin were available. These were too soft for many uses but we discovered that certain combinations of these metals mixed together made materials hard enough for everyday use.

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Alloying can also make dense metals lighter and expensive metals cheaper. Remember the story of Archimedes? This illustrates both ideas. According to legend, the king had given a lump of gold to a goldsmith to make a crown. When he received the crown, he suspected that the goldsmith had alloyed the gold with silver, which was cheaper and less dense. Archimedes was given the task of finding out if the gold had been alloyed. He couldn't damage the crown and needed a way of measuring the volume of the crown to check it's density. Apparently, when he noticed that the water in his bath rose when he got in, he realised that he could use the idea to get the volume of the crown and then work out the density.

Eventually, ancient humans discovered how to smelt more reactive metals like iron. Straight out of the smelter, iron is very brittle because of the impurities. But when it is purified and alloyed with carbon, it becomes more malleable and ductile. Weapons and tools made from iron superseded those made from bronze as they had better properties - the iron age had begun.

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  1. Aluminium is a light metal but why is it not used to make aircraft?
    Mixing it with small quantities of other metals increases its strength and durability
  2. Which of the following is a use of mild steel?
    Mild steel is used in cables for cranes because it remains very strong when being stretched. It is also used for the frames of large buildings and the construction of bridges because of its strength. It is used to reinforce concrete because it expands and contracts at the same rate as the concrete when heated and cooled, adding strength and flexibility
  3. What is an alloy?
    Quite often, there is only a very small percentage of the added metal or non-metal in an alloy
  4. Which of the following is not a reason for making an alloy?
    There are many more good reasons for making alloys instead of using just the metal, e.g. to make it stronger, more ductile or less dense
  5. How is the percentage of carbon in pig iron reduced to form steels?
    It is converted to carbon dioxide. At the same time, other impurities are removed
  6. Why are 'silver' coins made from an alloy or nickel plated steel and not from silver?
    Pure silver coins are easily defaced and lose weight rapidly because the silver gets rubbed away. The alloy used for making 'silver' coins is cupro-nickel
  7. Why are alloys stronger than the materials from which they are made?
    Metals are made up from layers of atoms that are able to slide past each other. Adding the different sized atoms makes it a lot harder for this to happen, thus increasing the metal's strength
  8. Which of the following is a use of high carbon steel?
    It is harder than mild steel and will stay sharper for longer as the carbon content is higher. However, the carbon makes it more brittle
  9. Why does iron directly from a blast furnace have limited usefulness?
    The impurities and the carbon make the iron very brittle. The impurities and at least half of the carbon have to be removed before it can be turned into anything useful. Iron direct from the blast furnace is called 'pig iron'
  10. Brass is an alloy of 70 per cent copper and 30 per cent zinc. Which of the following is not a use of brass?
    Brass conducts electricity well and is resistant to corrosion. It can be polished easily and resembles gold. It is hard to find something that hasn't been made of brass. Although some brass is used in aircraft, it is not normally part of the aircraft framework or skin as these require much lighter alloys

Author: Kev Woodward

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