Coloured reagents such as
litmus are termed indicators. A substance called phenolphthalein, a
coal-tar product, is a very delicate indicator; it is more sensitive to
acids than litmus is. Now there are some salts which contain a
preponderance of acid in their composition, _i.e._ in which the acid has
not been fully neutralised by the base; such salts are termed acid
salts. Bicarbonate of soda is one of these acid salts, but so feeble is
carbonic acid in its acid properties and practical evidences, that we
shall see both monocarbonate or "neutral" carbonate of soda and
bicarbonate or "acid" carbonate of soda show evidences of, or, as
chemists say, react with alkalinity towards litmus. However,
phenolphthalein, though reacting alkaline with monocarbonate of soda,
indicates the acidity of the bicarbonate of soda, a thing which, as I
have just said, litmus will not do. We will take two jars containing
solution of monocarbonate of soda, and in the first we will put some
phenolphthalein solution, and in the second, some litmus tincture.
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