There is a fundamental difference between a mere mixture and a chemical compound. If some lead shots be mixed with some powdered sulphur ever so carefully, they can be easily separated again. For instance, the sulphur can be blown away from the mixture by the use of a bellows, or all the shots can be picked out and removed one by one. Hut if, instead of removing the shots, the mixture is subjected to strong heat, both the lead and the sulphur disappear as such. They enter into combination with each other in a chemical sense, and a new substance or chemical compound is formed, named lead sulphide, which has properties or qualities quite different from those of its constituents. The tarnish which forms on articles of silver when exposed to the air of towns is also a chemical combination of silver and sulphur due to the presence of traces of a compound of sulphur in the air. Iron filings may be mixed ever so carefully with sand; but tnis, again, is a mere mixture, and all the iron filings may be separated or withdrawn from the mixture by means of a magnet, which attracts the iron and not the sand, showing that they are not in chemical combination, but only mechanically admixed. Gunpowder is only a mechanical admixture of nitre, charcoal, and sulphur, and their separate respective particles can be seen ly;ng apart from each other by means of the microscope. As a further instance of the difference between a mere mixture and a chemical compound it may be mentioned that when quicksilver is heatted and exposed to the air, it becomes changed into a yellow powder, which is a chemical compound of the mercury and atmospheric oxygen, and by no mere mechanical process can the mercury and the oxygen of which it is compounded be separated from each other, showing that it is not a mere mixture of the two things. Chemical combination always takes place in equivalent weights, or socalled combining proportions of the elements concerned, and the combining weights are the smallest which will combine with one part of hydrogen. Lead, for example, has an atomic weight of 207, so that when it enters into chemical combination with sulphur, which has an atomic weight of 32, 207 parts by weight of lead combine with 32 parts by weight of sulphur, and yield 239 parts of the chemical compound sulphide of lead. Salt is a chemical combination of 23 parts by weight of the metal sodium and 35 parts by weight of the gas chlorine, or, in oiher words, a combination of one atom of each of these two elements. The two atoms thus combined make up a molecule, so that the molecular weight of the compound is that of the two added togethernamely, 58J. When salt is decomposed that is, split up by chemical means into its two constituent elements58J parts by weight always yieid 23 parts by weight of sod

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