  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 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 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. OTHER METHODS OF SALT FORMATIONSolution of Metals in Acids. Alkalis are not the only substances which neutralize acids. Speaking in a broad and general sense, we may say that an acid is neutralized when a metal is dissolved in it, because, when the point is reached at which no more metal will dissolve, all the characteristic properties of the acid are destroyed. A salt is formed in this case also. An example will now be given to illustrate this method of salt formation. Before two pieces of metal can be united by soldering, it is necessary to clean the surfaces of the metal and the soldering iron. The liquid used for this purpose is made by adding scraps of zinc to Solution of Oxides in Acids. The substances most used in commerce with the express purpose of destroying acidity are quicklime, washing soda, and powdered chalk. Since quicklime is a compound of the metal calcium and the gas oxygen, its systematic name is calcium oxide; when it neutralizes an acid, it forms the corresponding calcium salt; for example, if it neutralizes acetic acid, calcium acetate is formed. An instance of the neutralization of an acid by an oxide of a metal is furnished by one method of preparing blue vitriol (copper sulphate). Copper does not dissolve very quickly in dilute sulphuric acid; hence, to make blue vitriol from scrap copper, the metal is first heated very strongly while freely exposed to air. Copper and oxygen of the air combine to form the brownish black powder, copper oxide, and this dissolves very readily in sulphuric acid, making the salt, copper sulphate. Solution of Carbonates in Acids. Washing soda and chalk belong to a different class of chemical substances. They are carbonates, that is, they are salts of carbonic acid. At first it may seem a little perplexing to the reader to learn that a salt can neutralize an acid to form a salt. It must be remembered, however, that acids differ from one another in strength, that is, in chemical activity, and that carbonic acid is a weak acid. When a salt of carbonic acid—sodium carbonate As an example of the neutralization of acids by carbonates, we may mention here a practical sugar saving device. Unripe fruit is very sour because it contains certain vegetable acids dissolved in the juice. These acids are not affected by boiling; and, therefore, to make a dish of stewed fruit palatable, it is necessary to add sugar in quantity sufficient to mask the sour taste. If a pinch of bicarbonate of soda is added to neutralize the acid, far less sugar will be necessary for sweetening. Insoluble Salts. The methods given above apply only to those salts which are soluble in water. Insoluble salts are obtained by mixing two solutions, the one containing a soluble salt of the metal, and the other, a soluble salt of the acid or the acid itself. The formation of an insoluble salt by the interaction of two soluble substances is well illustrated in the preparation of Burgundy mixture, the most effectual remedy yet proposed for checking the spread of potato disease. This mixture contains copper carbonate, that is, the copper salt of carbonic acid. For its preparation we require copper sulphate and sodium carbonate (washing soda), a soluble carbonate. When these two substances, dissolved in separate portions of water, are mixed, copper carbonate is formed as a pale blue solid which is in such a state of fine subdivision that it remains suspended in the solution of sodium sulphate, the other product of the reaction. The change is represented diagrammatically below. Each circle represents the atom or a group of atoms named therein. At the moment of mixing, these groups undergo re-arrangement. Bordeaux mixture, which some gardeners prefer, is a similar preparation containing copper hydroxide instead of copper carbonate. It is made by mixing clear lime water (a soluble hydroxide) with copper sulphate. Fig. 1 Fig. 1 Elements and Compounds. It is scarcely possible to discuss chemical processes without having from time to time to use terms which are not in everyday use. A few preliminary definitions and explanations of terms which will be frequently used may serve to simplify descriptions, and render it unnecessary to encumber them with purely explanatory matter. Among the many different kinds of materials known, which in the aggregate amount to several hundreds of thousands, there are about ninety substances which up to the present time have not been broken up into simpler kinds. These primary materials are called “elements,” the remainder being known as “compounds.” The following is a list of the commonest of these elements, together with the symbols by which they are represented in Chemistry.
The first step in the building-up process consists of the union of a metallic with a non-metallic element. Such compounds are binary compounds, and are distinguished by the termination -ide added to the name of the non-metallic element; for example, copper and oxygen unite to form copper oxide, sodium and chlorine form sodium chloride, iron and sulphur form iron sulphide or sulphide of iron. A compound containing more than two elements is distinguished by the termination -ate. Most salts fall within this category; thus we speak of acetate of lead and chlorate of potash, also of sodium sulphate and copper sulphate, the latter form being the more correct. A difficulty arises when two bodies are composed of the same elements combined in different proportions. Then we have to resort to other distinguishing prefixes or suffixes. For this reason we meet with sulphurous Crystals and Water of Crystallization. When a soluble salt is to be recovered from its solution, the latter is reduced in bulk by evaporation until, either by experience or by trial, it becomes evident that the solid will be formed as the liquid cools. In some cases, when time is not an important factor, evaporation is left to take place naturally. Under either set of conditions, the substance generally separates out in particles which have a definite geometrical form. These are spoken of as crystals. Crystals often contain a definite percentage of water, called “water of crystallization.” In washing soda, this combined water forms nearly 63 per cent. of the total weight; in blue vitriol, it is approximately 36 per cent. On being heated to a moderate temperature, the water is expelled from the solid; the substance which is left behind is called the anhydrous (that is, the waterless) salt. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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