ILLUSTRATIONS
FIG. PAGE [BACTERIA NODULES ON THE ROOT OF LUPIN] Frontispiece [1. DIAGRAM] 7 [2. PLAN OF SULPHURIC ACID WORKS] 13 [3. GENERAL VIEW OF SULPHURIC ACID WORKS] 15 [4. SULPHUR TRIOXIDE—THE CONTACT PROCESS] 19 [5. PREPARATION OF NITRIC ACID] 30 [6. NITROGEN CYCLE (DIAGRAM)] 38 [7. NITRIC ACID FROM AIR (DIAGRAM)] 41 [8. PREPARATION OF HYDROCHLORIC ACID] 45 [9. BORIC ACID] 59 [10. QUICK VINEGAR PROCESS] 71 [11. DUTCH PROCESS FOR WHITE LEAD] 74 [12. SALT CAKE FURNACE] 83 [13. BLACK ASH FURNACE] 85 [14. THE SOLVAY PROCESS] 89 [15. THE ELECTROLYSIS OF SALT SOLUTION] 102 [16. THE CASTNER PROCESS] 105
ACIDS, ALKALIS, AND SALTS
CHAPTER I
INTRODUCTION
Acids. A vague hint from Nature gave mankind the first indication of the existence of acids. The juice pressed from ripe grapes is a sweetish liquid. If it is kept for some time, the sweetness goes, and the liquid acquires a burning taste. If kept still longer, the burning taste is lost, and in its place a sharp acid flavour, not entirely displeasing to the palate, is developed. The liquid obtained in this way is now called wine vinegar; the particular substance which gives it its characteristic taste is acetic acid.
The strongest vinegar does not contain more than 10 per cent. of acetic acid, which is itself a comparatively weak acid. It is, therefore, not a very active solvent. Nevertheless, for metals and for limestone rock, and other substances of a calcareous nature, its solvent power is greater than that of any other liquid known at the time of its discovery. It was this property which seems to have appealed most strongly to the imagination of the early chemists; and, as is very often the case, the description of its powers was very much exaggerated. Livy and Plutarch, who have given us an account of Hannibal’s invasion of Italy by way of the Alps, both gravely declare that the Carthaginian leader cleared a passage for his elephants through solid rocks by pouring vinegar over them!
In the Middle Ages, the study of Chemistry was fostered mainly as a possible means whereby long life and untold riches might be obtained. The “Philosopher’s Stone,” by the agency of which the base metals were to be changed to gold, and the “Elixir of Life,” which was to banish disease and death, were eagerly sought for. Though these were vain imaginings according to modern ideas, nevertheless they were powerful incentives towards experimental work. Many new substances were discovered in this period, and among these were nitric acid (aqua fortis), hydrochloric acid (spirit of salt), and sulphuric acid (oil of vitriol).
Acids were then valued above all other substances. The mediaeval chemist (or alchemist, as he was called) clearly saw that unless a body could be dissolved up there was no hope of changing it. Nitric acid, therefore, which, in conjunction with hydrochloric acid, dissolved even gold itself, was very highly esteemed. Oil of vitriol also was scarcely less important, for it was required for the production of other acids.
So far, taste and solvent power were considered to be the characteristic feature of acids. In the time of Robert Boyle (1627-1691), they were further distinguished from other substances by the change which they produced in the colour of certain vegetable extracts. Tincture of red cabbage was first used, but, as this liquid rapidly deteriorates on keeping, it was soon replaced by a solution of litmus, a colouring matter obtained from Roccella tinctoria and other lichens. It imparts to water a purple colour, which is changed to red by the addition of acids.
Alkalis. Wood ashes were valued in very early times because they were found to be good for removing dirt from the skin. Mixed with vegetable oil or animal fat, they formed a very primitive kind of soap, which was afterwards much improved by using the aqueous extract instead of the ashes themselves, and also by the addition of a little caustic lime.
When plant ashes are treated with water, about 10 per cent. dissolves. If the insoluble matter is then allowed to settle down and the clear liquid evaporated to dryness, a whitish residue is obtained. The soluble matter thus extracted from the ashes of plants which grow in or near the sea is mainly soda; that from land plants, mainly potash. Formerly no distinction was made, and the general term “alkali” was applied to both.
In order to bring the properties of alkalis into contrast with those of acids, we cannot do better than make a few simple experiments with a weak solution of washing soda. Its taste is very different from that of an acid; it is generally described as caustic. If a little is rubbed between the fingers, it feels smooth, almost like very thin oil. It does not dissolve metals or limestone. Its action on vegetable colouring matter is just as striking as that of acids. Tincture of red cabbage becomes green; the purple of litmus is changed to a light blue. This colour change is characteristic of alkalis.
Neutralization. When the colour of litmus solution has been changed to red by the addition of an acid, the original colour can be restored by adding an alkali. The change can be repeated as often as desired by adding acid and alkali alternately. From this we get a distinct impression of antithesis between the two. In popular language, an alkali “kills” an acid; in Chemistry, the same idea is expressed by the term “neutralization.”
Salts. Both “neutralization” and “killing the acid” are modes of expression which describe the phenomenon fairly well. When an acid is neutralized, its characteristic taste, its solvent power, and its action on litmus, are all changed; in fact, the acid as an acid ceases to exist, and so does the alkali. When the neutral solution is evaporated to dryness, a residue is found which on examination proves to be neither the acid nor the alkali, but a compound formed from the two. This substance is called a salt.
To most people, salt is the name for that particular substance which is taken as a condiment with food. Its use in this connection dates from time immemorial. It is distinctly unfortunate that another and very much wider usage of the term has been introduced into Chemistry. When the early chemists recognized that other substances, which they vaguely designated as “saline bodies,” were similar to common salt in composition, they took the name of the individual and applied it to the whole class.