Metal alloys Metals are of interest not just in their pure state, but also because some can form alloys — some atoms of one metal are substituted for some atoms in the lattice of another metal (substitutional alloys). The alloy has different properties from the original metals, for example: ●●
colour (as in the case of zinc and copper forming brass)
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hardness (again with brass)
Another example is the use of a non-metal (carbon) as well as other metals to Hodder CIE revision guide 2010 Chemistry fig 4.8a and 4.8b produce the variety of specialist steels now available. Aluminium alloys have 18 Aug 2010 become increasingly important as the demand for lightweight, strong materials has Eleanor Jones increased. Figure 4.8 shows the difference between a typical alloy, such as brass, and a steel (interstitial alloy), where the small carbon atoms fit between metal atoms.
(a)
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Cambridge International AS and A Level Chemistry Revision Guide
(b)
Substitutional atom
Interstitial atoms
Figure 4.8 Structure of (a) a typical alloy and (b) a steel
Metals have to be mined and separated from the waste rock. Today, most of the ores rich in metals have already been mined and we are using ores that contain much less metal. Under these circumstances it makes sense to try to recycle metals because new extraction from low-grade ores requires huge amounts of energy.
Try this yourself (18) Study the data given about material X. Suggest, with reasons, the type of structure and bonding present in X. ●● X, a soft waxy solid, melts at just under 100 °C. ●● It is an electrical insulator, both as a solid and when molten. ●● It dissolves in cyclohexane to give a solution that does not conduct electricity.
Ice and water Water is an extremely unusual compound. For example, from its relative molecular mass (18) we would expect it to be a gas (compare with nitrogen (28), oxygen (32) and carbon dioxide (44)). It is the presence of relatively strong hydrogen bonds that make it a liquid. Without these, life would not exist. The boiling points of some hydrogen compounds are compared in Figures 4.9(a) and (b).
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