The Romance of Modern Chemistry

FACTS ABOUT SOLUTIONS a comparatively small resistance. The addition of salt confers on water the power to conduct the electric current, and the salt solution may be electrolysed as described in the previous chapter. The case of sugar is quite different. The resistance offered by water to the passage of a current is not diminished by the addition of sugar, nor can a solution of sugar be electrolysed. Such differences in behaviour as those exhibited by salt and sugar are shown by hosts of other substances. Of the chemical compounds which can be dissolved in water, many, comprising acids, bases, and salts, behave like sodium chloride, and are accordingly called “ electrolytes ”; the others, which, like sugar, do not increase the conducting power of water, are known as “non-electrolytes.” Now the curious thing is that it is just those substances which make water a conductor—that is, the electrolytes— which have an unexpectedly big effect on the freezing- point of water. This coincidence was emphasised some twenty-five years ago by the well-known Swedish chemist Arrhenius, who also suggested an ingenious explanation. According to this, the molecule of an electrolyte, when dissolved in water, is liable to “ dissociate,’1 or split up into two parts or “ ions,” one of which carries a positive electric charge and the other a negative charge. When, for instance, sodium chloride is added to water, the atom of sodium and the atom of chlorine which have combined to form the molecule of that substance, are instantly seized with a desire for divorce, and they separate, so far, at least, as the most of the molecules are concerned, giving rise to a positively charged sodium ion and a negatively charged chlorine ion. At first this theory seems rather fantastic; there 311