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107

Stories About Chemistry

INDEX

28. Unusual Compounds

What was the first chemical compound deliberately produced by man?

The history of science can give no definite answer to this question. Let us take the liberty to make our own assumptions on this point. The first substance which man prepared, knowing ‘beforehand what he wanted to obtain, was a compound of two metals, copper and tin. We have deliberately avoided using the word “chemical, “ because the compound of copper and tin (commonly known as bronze) is an unusual one. It is called an alloy.

The ancients first learned to smelt metals from their ores and only afterwards to fuse them with each other.

Thus, at the dawn of civilization appeared the first seeds of a branch of the future science of chemistry, now called metal chemistry.

The structure of compounds of metals and nonmetals usually depends on the valence of the elements contained in them. For example, the molecule of common salt contains positively univalent sodium and negatively univalent chlorine. In the ammonia molecule NH3 negatively trivalent nitrogen is linked with, three positively univalent hydrogen atoms.

The chemical compounds of metals with one another (called intermetallic compounds) usually do not obey the laws of valence, and their composition bears no relation to the valence of the reacting elements.

For this reason the formulas of intermetallic compounds look rather strange, for instance, MgZn5, KCd7, NaZn12, etc. The same pair of metals can often give several intermetallic compounds; for instance, sodium and tin form nine different combinations.

Metals interact with one another in the molten state, as a rule. But metals do not always form chemical compounds with each other when fused. Sometimes one metal simply dissolves in the other. The result is a homogeneous mixture of indefinite composition which cannot be expressed by any distinct chemical formula. Such a mixture is called a solid solution.

Alloys are legion; nobody has ever taken the trouble to count up even approximately how many of them are known already and how many can be obtained, in general. As in the case of organic compounds, this figure would probably also run into the millions.

There are some alloys which consist of no less than a dozen metals, and each new addition has a specific effect on their properties. There are many alloys which contain only two metals, these being called bimetallic, but their properties depend on the proportion of their components. Some metals fuse very readily and in any proportion. Such are bronze and brass (an alloy of copper and zinc). Others, such as copper and tungsten, are reluctant to mix under any conditions. Still, scientists have succeeded in making an alloy of them, though in an unusual manner, by what is known as powder metallurgy, that is, by sintering copper and tungsten powders under pressure.

Some alloys are liquids at room temperature; others are very resistant to high temperatures. The latter are used in large quantities in space engineering. Finally, there are alloys which do not yield even when attacked by the strongest chemical agents, and alloys which are almost as hard as diamond.


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