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J.] CHARACTERS OF CHEMICAL ATTRACTION. 3
greatly resembling them which are but sparingly soluble, such as lime and baryta; these are termed alkaline earths. There are also numerous other analogous substances, compounds of oxygen with the metals, termed oxides, such for instance as the compound of silver with oxygen, or argentic oxide, the compound of iron with oxygen, or ferric oxide, and that of lead with oxygen, or plumbic oxide, which are insoluble in water, but which are easily dissolved by acids, forming on evaporation crystalline compounds or salts. For instance, by the action of nitric acid upon argentic oxide, argentic nitrate and water are produced* (i); with sulphuric acid and ferric oxide, ferric sulphate and water are the products (2); whilst with acetic acid and plumbic oxide, plumbic acetate and water are obtained (3). Any substance, whether soluble in water or not, is called a base if it thus have the power of reacting with acids, neutralizing their properties, and furnishing by such action a salt, whilst at the same time water is formed. Hence the alkalies constitute one subdivision of the more numerous class of bodies known as bases. (7) Characters of Chemical Attraction.—Chemical attraction is distinguished by well-marked characters from other kinds of action which take place within minute distances.
* A metal which by its union with oxygen produces a base is designated as a batyl. Argentic oxide, or oxide of silver, for example, is a base, whilst silver itself is a basyl. So also plumbic oxide is a base, whilst lead is regarded as B basyl, when we refer to its occurrence in its salts. In two similar manner water is itself, when compared with acids, sometimes spoken of as the base, whilst hydrogen is a basyl. When a base containing oxygen acts upon an acid, which, as has already been defined, is a salt of hydrogen, the hydrogen in the acid and the metal in the base change places, a metallic salt is formed, and water is set at liberty, as is shown by the following symbols and equations, which illustrate the formation of argentic nitrate, ferric sulphate, and plumbic acetate respectively :—
(1) Ag,0 +2HN0, = Hs0+2AgN0,
(2) Fe2U, + 3 H,S04 = 3 H..O + Fe, (S04),
(3) PbO + 2 HC.U.0,|= H,0 + Pb (0,11,0,),.
The explanation of the principle upon which these symbols are constructed will be found at p. 23.
We meet not unfrequently with bases in which hydrogen as well as a basylous metal is present, e.g. in caustic soda (Na HO), or slaked lime (Ca H202). Salts are also formed by the action of such compounds upon the acids, and at the same time the hydrogen both of the acid and of the base is separated in the form of water. This, for example, is what occurs when hydrated soda is neutralized by hydrochloric acid, common salt and water being the products; for instance:—
Na HO + HC1 = H,0 + Na CI;
or when slaked lime is converted into calcic sulphate by sulphuric acid:—
Ca 11,0, + H2S04 = 2H,0 + Ca SCy
10 DIFFERENCE BETWEEN MIXTURE Cr.
i. Chemical attraction is exerted within its own limits with intense energy; but beyond those limits it is entirely powerless. An iron wire, for example, which will support a weight of iooolb., will in a few minutes yield to the almost noiseless action of a mixture of nitric acid and water; the stubborn metal will be dissolved, and a clear solution of the metallic mass will be formed: the metal will be detached from the wire particle by particle, and no vestige of its structure or tenacity will remain. It is rarely possible, by trituration or other mechanical means, to bring about a sufficient approximation amongst the subdivided particles to produce chemical action. Tartaric acid and sodic carbonate, each in the form of a dry powder, may be incorporated by grinding for hours in a mortar, but they will not act chemically upon each other: it is not until a more intimate contact is effected by the addition of water, which dissolves the particles of both, and allows them mutually to approach closer, that the brisk effervescence, due to the expulsion of the carbonic anhydride, occurs, which indicates the transfer of the sodium with which it was previously in combination, to the radicle of the tartaric acid, and the formation of water and sodic tartrate.
It is important to distinguish clearly between a mere mechanical mixture and a chemical compound, for the effects produced by mixture and by combination are very different. In a mixture the proportions of the materials may be varied at pleasure, and the properties of the mixture will partake of those of each of its constituents; whereas in a chemical compound the proportion of each of its constituents is invariable, and the properties of the compound are nearly always quite different from those of the bodies which have entered into its formation.
A striking illustration of the difference between the effects of mechanical intermixture and those of chemical combination is afforded in the case of ordinary gunpowder. In the manufacture of this substance, the materials of which it is made—viz., charcoal, sulphur, and nitre—are separately reduced to a state of fine powder; they are then intimately mixed, moistened with water, and thoroughly incorporated by grinding for some hours under edge stones; the resulting mass is subjected to intense pressure, and the cakes so obtained, after being broken up and reduced to grains, furnish the gunpowder of commerce. In this state it is a mechanical mixture of nitre, charcoal, and sulphur. The nitre may be washed out of the mixture by means of water, the sulphur by means of carbonic disulphide, while the charcoal will be left undissolved. By evaporating the water, the nitre may be
7.] AND CHEMICAL COMBINATION. 11
obtained; and on allowing the disulphide to volatilize, the sulphur will be left behind. If, however, we cause the materials to act chemically on one another, all is changed:—a spark fires the powder; the dormant chemical attractions are called into operation, the charcoal disappears, a large volume of gaseous matter is liberated, and new substances are produced, which have no resemblance to the original mixture.
ii. Chemical attraction is most strongly exerted between dissimilar substances.* No manifestation of this attraction takes place between two pieces of iron, two pieces of copper, or two pieces of sulphur; but between sulphur and copper, or sulphur and iron, chemical action of the most energetic kind may occur. It is highly probable, however, as will be seen hereafter, that in many cases the isolated bodies usually viewed as elements should be regarded as compounds, the molecules of which consist of particles of the same element in opposite polar or electrical conditions. Hydrogen gas, for instance, is to be regarded as hydride of hydrogen, or a compound of hydrogen with hydrogen; chlorine gas, as chloride of chlorine, and so on.
Many compound bodies, when subjected to the action of the voltaic current in a liquid state, are decomposed, one of the elements going to the positive electrode of the voltaic battery, the other to the negative electrode. The elements have hence been divided into two groups according to their electrical relations, those which are separated at the positive electrode being termed electro-negative, or chlorous elements, which include such bodies as chlorine, iodine, oxygen, and sulphur; whilst those which are found at the negative electrode are termed electro-positive or basylous elements (261), among which are comprised potassium, sodium, and other metals which, when united with oxygen, form bases.
Generally speaking, the greater the difference in the properties of the two bodies, the more intense is their tendency to mutual chemical action. Two electro-positive elements may combine, as may also two electro-negative elements; but such compounds are much less stable than those formed by the union of an electro-positive with an electro-negative element. For instance, the metals, as a class, combine readily with chlorine, oxygen, and
q This circumstance—viz., that the elements which combine are not allied in properties—is sufficient to indicate the objection to the use of the term affinity, to express this form of attraction, although custom has sanctioned its employment.
12 ORDER OF ATTRACTION. [7.
sulphur:—silver unites rapidly with sulphur, iron with chlorine, potassium with oxygen, and so on. But between bodies of a similar character the tendency to unite is feeble. For example, chlorine and oxygen can only be made to combine by indirect methods, and when combined, a slight elevation of temperature is sufficient to cause them to separate from each other. In like manner, two metallic bodies, copper and zinc, will, under the influence of a high temperature, unite and form brass,—an alloy, the properties of which indicate that it may be regarded as a chemical compound; but brass, on being heated strongly, may be separated into zinc, which nearly all passes off in vapour, and copper, which is left behind.
Even in bodies of the same class chemical attraction is exerted between different kinds of matter with different but definite degrees of force. Nitric acid will dissolve most of the metals— such, for instance, as silver, mercury, copper, and lead—and by so doing will produce a nitrate of each metal; but the chemical compounds thus formed are held together with very different degrees of energy. The combination with silver at ordinary temperatures is less powerful than with mercury, less so with mercury than with copper, and with copper less again than with lead.
This fact may easily be determined by dissolving half an ounce of argentic nitrate (nitrate of silver) in half a pint of water, and pouring into it a small quantity of clean mercuryi in the course of a few days a beautiful crystallization of metallic silver will be obtained, whilst a corresponding quantity of mercury will be dissolved, and will combine with the nitrion (N03) previously in union with the silver. In a similar manner mercury may be displaced from a solution of mercurous nitrate by means of a strip of metallic copper; and copper, in its turn, may be displaced by a piece of lead introduced into a solution of cupric nitrate. From a solution of plumbic nitrate, zinc will, in like manner, displace the lead, which will be deposited in beautiful crystals.
Guided by facts like these, different elements have been arranged in tables indicating the order of their attraction for any one element which is placed at the head of the list. For example, in the first column of the following table, several of the more important metals are arranged in the order of their tendency to combine with oxygen,—the metal at the head of the list having the strongest attraction for that element, that which stands second the next, and so on to the one mentioned last, in which the attraction is the weakest:—
MATTER NOT DESTROYED BY CHEMICAL ACTION. 13
Order of Displacement from Solution.
OXYGEN. SULPHURIC ACID.
Mercury. Zincic oxide.
Similar tables may be formed, exhibiting the relative tendency of compound bodies, such as bases, to displace each other from the salts which they have contributed to form. In the second column of the table, the various bases are arranged in the order in which they displace each other at ordinary temperatures from the salts which they form when acted on by sulphuric acid.
iii. Another of the most remarkable and characteristic features of chemical attraction is the entire change of properties which it occasions in both the substances dealt with,—a change which no a priori reasoning could possibly have predicted. If the blue liquid obtained by dissolving copper in nitric acid be evaporated, there will be formed a blue crystalline salt which has none of the chemical properties either of the acid or of the metal from which it was procured. There is an equally striking difference between the tough, metallic, insoluble iron, and the corrosive oil of vitriol, and the beautiful crystalline, green, soluble, inky-tasted salt which is produced by their mutual action.
iv. The experiments on the displacement of one metal by another, further show that although in combination, the properties of the components are masked, and to all ordinary observation the constituents have disappeared, whilst new bodies have been formed, yet these component elements do really exist in the compound, and can be again reproduced in their original form by separating them from each other.
It is, indeed, a general principle, that whenever chemical combination occurs, there is no destruction of matter. However much the materials may change their form, the weight of the new products, if collected and examined, will be found to be exactly equal to that of the substances before combination. Thefollowing experiment shows that, even although the substance may vanish from our sight, it continues to exist as a gas, which has the same weight