In the present state of chemical science a satisfactory definition of the term salt cannot be given. The older chemists regarded a salt as a product of the "union" of an acid with a base, as when (using the older notation, as well as atomic weights) nitric acid (NO5) unites with potash (KO) to form nitrate of potash (KO,NO5); and this definition is often used at the present time, but according to modern theory it is not strictly correct. To say that a salt is produced by the "action" of an acid on a base is correct as far as it goes, but salts are sometimes formed by the direct union of two elements, neither of which is an acid or a base. By the term base is meant a body composed of two or more elements (inorganic bases usually having only two), most frequently an oxide of a metal, which is capable of effecting a double decomposition with an acid, during which water and a salt are formed by the exchange of elements, as when oxide of silver is acted upon by nitric acid (Ag2O + 2HNO3=2AgNO3 + H2O),. where the oxygen of the oxide of silver unites with the hydrogen of the nitric acid to form water, while the metallic basyle silver unites with the radical (NO3) to form nitrate of silver.
This is a different action from that formerly supposed to take place, that is, the direct union of the base and acid, without double decomposition; for instance, using the old notation and equivalent numbers, AgO + NO5=AgO, NO6. That part of the base which unites with a portion of the acid to form the salt is usually called the basyle. In the formation of nitrate of silver above described, the basyle Ag of the base Ag2O displaces the hydrogen which in the nitric acid is united with the radical NO3. Lavoisier supposed that all true acids contained oxygen, and gave the name (meaning acid generator) to that element in accordance with that hypothesis. The term acid was applied to both the anhydrides and their compounds with water, which latter are now regarded as the only true acids. In course of time it was discovered that there were acids containing no oxygen, such as hydrochloric, hydriodic, and hydrobromic acids, which possessed all the other characteristics of Lavoisier's acids, as sourness to the taste and the power to redden vegetable blues.
This led to the division into oxyacids and hydracids; but it was found that the constitution of common salt was simply binary, it being composed of the metallic basyle sodium united to the element chlorine, and having the formula NaCl. Berzelius then propounded the theory that a salt consisted of an electro-positive body united to an electro-negative body, each of which might be either simple or compound. When simple, as in common salt, they formed haloid salts, so named from their resemblance to common salt (Gr. , sea salt), and consisted of an electro-positive metal united to an electro-negative radical or halogen. When these bodies were compound they formed amphide salts, and these amphide salts might contain oxygen in both base and acid, or they might contain sulphur in both; in one case being called oxy-salts, and in the other sulpho-salts. The haloid salts were strictly binary compounds, but the amphide salts were regarded as ternary. Davy and Dulong also introduced a theory by which the seeming difference between oxyacid and hydracid salts was reconciled. This was called the binary theory, and it regarded all hydrated acids as in reality salts, containing hydrogen in place of a metal, and acting the part of a basyle toward a single element or group of elements, and all salts as being built up on the type of chloride of sodium. Thus sulphuric acid, H2SO4, may be regarded as a salt similar in constitution to potassic sulphate, K2SO4, the only difference being the presence of the feeble basyle hydrogen in place of the powerful basyle potassium. By the action of sulphuric acid on zinc there is simply displacement of hydrogen by zinc (H2SO4 + Zn=ZnSO4+2H), hydrogen being evolved in a gaseous state.
According to the old ideas, using the old notation, sulphuric acid, H2O,SO3, acting on the zinc, caused electric polarization, by which the affinity of the metal for oxygen was so increased that it rapidly decomposed water, liberating the hydrogen, forming a base, ZnO, with the oxygen, and then uniting with the anhydride SO3, forming sulphate of zinc, ZnO,SO3. The binary theory, it will be observed, simplifies the reactions, at the same time that it admits of the agency of the electro-motive force; for in the composition of sulphate of zinc, ZnSO4, the metal is regarded as an electro-positive basyle, while the body SO4 is regarded as an electro-negative radical, composed of the anhydride of the acid plus oxygen, and called generically an oxion. When the oxygen is united to sulphur, as in sulphuric acid, the oxion is specifically called a sulphion, or sometimes an oxysulphion. In the case of sulphurous acid, instead of being a sulphion it is a sulphosion. When the radical contains nitrogen instead of sulphur, it is an oxion, which is specifically called a nitrion or a nitro-sion, according as it is a constituent of nitric or nitrous acid.
The objections to the binary hypothesis are, that none of the compound radicals or oxions, SO4, NO3, or CO3, have ever been isolated; and it also appears improbable that a compound which is held together by such powerful attractions as in potash exist between potassium, the most highly electro-positive, and oxygen, the most highly electro-negative element, should be decomposed by the action of carbonic anhydride, CO2, parting with its oxygen, so that K2O + CO2 should become K2,CO3 instead of K2O,CO2. Chemists are now more inclined to regard a salt, when once formed, as a whole, and not as consisting of two distinct parts, although it is probable that during the act of combining the electro-chemical relations of the constituents are distinct and opposite. It is sometimes convenient, however, to regard salts as having the constitution of binary compounds, and as consisting of a basyle and a radical constantly held together by opposite electric polarities. It was formerly supposed that salts are formed only between acids and bases of the same class; that is, that both members must be oxides, sulphides, chlorides, etc.; and this was consistent with the ternary hypothesis, which regarded the salt as a combination of a compound base with an anhydride and not with its radical, as has been above illustrated.
Upon this hypothesis it will be seen that sul-phuret of potassium could not form a salt with sulphuric acid. It required decomposition to effect this, including the evolution of sulphuretted hydrogen gas, thus: K2S+H2SO4= K2SO4+H2S. But according to the binary theory a similar reaction takes place on the addition of sulphuric acid to oxide of potassium, water instead of sulphuretted hydrogen being formed: K2O+H2SO4=K2SO4+H2O. - There are three varieties of salts which depend upon the relative proportions of radical to basyle, or, in common language, of acid to base. They are called neutral or normal, acid, and basic or subsalts.
A salt is commonly said to be neutral when the characteristics of both acid and base have neutralized each other, and this condition is usually regarded as existing when the salt has neither the effects of acids nor alkalies upon certain vegetable colors. The blue color of litmus is changed to red by an acid, and again restored by an alkali, while a perfectly neutral salt produces neither of these effects. The yellow of turmeric is turned brown by an alkali, and is restored by an acid. But there are some salts which are regarded as neutral in composition, or, to use a more appropriate term, normal, which have the power of changing vegetable blues to red, and vice versa. There are some acids (and they are all now regarded as salts of hydrogen) that contain only one atom of hydrogen which can be displaced by one atom of a monad metal. Such acids are said to be monobasic, and among them are hydrochloric, HCl, nitric, HNO3, and acetic, HC2H3O2. When these acids unite with bases, they are capable of forming only monobasic salts, that is, salts containing one atom of ba-syle. Other acids contain two atoms of hydrogen, which may be replaced by two atoms of a monad metal like potassium, or one equivalent of a dyad like zinc.
These acids are called dibasic, and among them are sulphuric, H2SO4, and tartaric, H2C4H4O6. Other acids again contain three atoms of hydrogen, which may be replaced by three atoms of a monad metal, or one atom of a triad; and such acids are said to be tribasic, of which tribasic phosphoric acid, H3PO5, and citric acid, H3C6H5O7, are examples. Acids and salts which contain more than one equivalent of basyle are said to be polybasic. In general it may be said that when all the atoms in the hydrogen basyle of the acid are, in the formation of the salt, replaced by an equivalent number of atoms of the metallic basyle, the salt as formed will be normal, or, in common language, neutral; although it must be remembered that some normal salts will change vegetable colors.
When the atoms of the hydrogen basyle are only partially replaced by a metallic basyle, the salt so formed is an acid salt, the acid character of the hydrogen compound (acid or salt) not having been neutralized by an equivalent of metallic basyle. The formation of a true acid salt therefore requires a polybasic acid, for if the one basyle of hydrogen in an acid is replaced by one metallic basyle, the salt so formed will be normal. An example of an acid salt is bisulphate of potassium (hydric-potassic sulphate), KHSO4, where only half of the basyle hydrogen is displaced, and there is only one atom of potassium instead of the two which are required to replace the hydrogen in sulphuric acid, H2SO4. Other examples are the organic salts, bitartrate of potassium (cream of tartar, hydric-potassic tartrate), KHC4H4O6, and bicarbonate of potassium (hydric-potassic carbonate), KHCO3. The normal salts corresponding to these are: potassic sulphate, K2SO4; potassic tartrate, K2C4H4O6; and potassic carbonate, K2CO3.
These are such as contain a greater number of atoms of metallic basyle than there were atoms of hydrogen basyle in the acid. An example of such salts is basic mercuric sulphate (turpeth mineral), HgSO4,2HgO, which contains three atoms of mercury in place of the two atoms of hydrogen that were contained in the sulphuric acid from which the salt was formed. The theory of the formation of basic salts is imperfect, and it will he observed that here there is not that complete replacement of basyle which exists in neutral and acid salts. The tendency to the formation of basic salts is limited to certain acids and bases. The monad basyles do not form basic salts. The dyad metals, such as copper, lead, and mercury, have a strong tendency to do so, while the triads, as antimony and bismuth, have a still stronger tendency.
In considering polybasic acids and salts, it was seen that one of the atoms of the hydrogen basyle of a dibasic acid might be replaced by an atom of a monad metallic basyle. Such an acid salt may be regarded as a true double salt of a metal and hydrogen. But a normal double salt may be formed by replacing one half of the hydrogen basyle with one monad metal, and the other half with another monad metal. Such are called double salts, of which Rochelle salt (tartrate of potash and soda), KNaC4H4O6 + 4Aq, is an example. Most of the double salts have this constitution, but others have a different formation. A remarkable class of double salts was investigated by Graham. In many cases the water of crystallization may be expelled from a salt by the temperature of boiling water; in other cases all but one molecule will be thus expelled, which requires a considerably higher heat. It was found that this last molecule of water could be replaced by a molecule of certain anhydrous salts. The formation of a certain class of sulphates illustrates this action. All the sulphates of metals isomorphous with magnesium are capable of forming double salts of this character with some anhydrous sulphate not isomorphous with this class, as potassic sulphate.
If magnesic sulphate, MgSO47H20, which parts with six of its molecules of water at 212° and crystallizes in right rhombic prisms, and potassic sulphate, K2SO4, which crystallizes in six-sided prisms, or in four-sided right rhombic prisms, are separately dissolved in water in equivalent proportions and mingled while at a temperature a little above 212°, the solution will deposit on cooling a new double salt, MgSO4,K2SO4 + 6H2O, having the same crystalline form as magnesic sulphate, but containing six instead of seven molecules of water of crystallization, potassic sulphate occupying the place of the seventh molecule. This seventh molecule has been termed by Graham saline water. Another well known variety of double salts are the alums, of which common potash alum, K2C124SO4 + 24H2O (or K2SO4, A123SO4 + 24H2O), is an example. (See Alum.) Haloid salts unite with each other to form double salts, the most common of which are formed by the chlorides, iodides, and bromides of the less oxidizable metals, and the alkaline and earthy metals.
Examples of such double haloid salts are the double chloride of potassium and platinum, 2KCl,PtCl4, and the double iodide of potassium and mercury, 2KI,HgI2. There is a class of salts called oxychlorides, oxybromides, oxycyanides, etc, in which one molecule of the chloride, of the bromide, or of the cyanide is united with one or more molecules of the oxide of the same metal, as in Turner's yellow, PbCl2,7PbO. - When any acid is added to the solution of a salt the basyle of which is capable of forming a soluble salt with the radical of such added acid, a partial exchange between the basyle of the salt and the hydrogen basyle of the added acid is supposed to take place, probably in the proportion of the relative attractions of these basyles for each radical. But if the radical of the added acid is capable of forming an insoluble salt with the basyle of the salt, the latter will be entirely decomposed, and its radical appropriated by the radical of the added acid; for as fast as the basic sulphate is formed it is removed from the solution by precipitation, which necessitates a continual decomposition of the first salt: Ba2NO3 + H2SO4=BaSO4 + 2HNO3. Similar reactions take place on adding a base to a saline solution.
If both bases and the salts which they form with the radical of the salt are soluble, the solution will remain clear; but if the added base forms an insoluble salt with the radical of the salt, the latter will be decomposed, while the new salt will be precipitated; or if the base of the salt be insoluble while the added base is soluble, a soluble compound will be formed, and the base of the first salt will be precipitated. Most of the metallic salts, with the exception of those of the alkalies and the alkaline earths, are formed from bases which are insoluble in water; consequently the addition of a soluble base, as potash or soda, to such metallic salts causes the precipitation of the base or oxide, and upon this reaction depend many of the chemical tests for metallic substances. Oxide of zinc, or zinc white, although prepared for commerce by distilling zinc into chambers supplied with currents of air, may be formed by precipitation from solution of its salts by an alkaline hydrate, for instance, from the sulphate by the action of potassic hydrate (ZnSO4 +K2O=ZnO+K2SO4), potassic sulphate being formed and remaining in solution. The nature of the double decomposition which takes place when two salts are brought together depends often upon the condition in which they are.
For instance, if ammonic sulphate and calcic carbonate are mixed together in a dry state and gently heated, decomposition takes place, and calcic sulphate and ammonic carbonate are produced, the latter being expelled as a volatile product: CaCO3 + (H4N)2SO4=(H4N)2CO3 +CaSO4. But if a solution of calcic sulphate and ammonic carbonate are mixed, the effects will be reversed, and calcic carbonate and ammonic sulphate will be formed, the former being precipitated, the latter held in solution.