4 H.SO, (S=32, O=16). Let the white solid obtained by evaporating the solution in water of potassium oxide be analysed; its composition is expressed by the formula KOH (K = 39, O=16). Each of these solids is then dissolved in water; the solution of H,SO, has a very sour taste, corrodes animal and vegetable membranes, and turns blue litmus bright red; the solution of KOH has a burning, but not a sour, taste, corrodes the skin, and turns red litmus deep blue. The compound H2SO, is an acid, the compound KOH is an alkali. 4 A measured portion of the aqueous solution of H2SO, is placed in a basin, a drop or two of litmus solution is added, and the solution of KOH is run in from a graduated vessel, drop by drop, with constant stirring, until the red colour of the litmus changes to a purplish-blue tint. The liquid is then evaporated to dryness, and the white solid which remains is analysed; its composition is expressed by the formula K2SO. This solid is dissolved in water; the solution has no pronounced taste, it is without action on vegetable or animal membranes, and it does not affect the colour of either blue or red litmus. A comparison of the compositions of the three compounds (1) KOH (2) H2SO (3) K.SO. shews that (3) differs from (2) in that hydrogen does not enter into its composition, but in place of two combining weights of hydrogen-combined with one combining weight of sulphur and four combining weights of oxygen-there are two combining weights of potassium. Compound (1) is an alkali, (2) is an acid, (3) is a salt. The salt is produced by the interaction of the alkali and the acid. The interaction in question is represented by the equation 2KOHAq + H2SO1Aq = K2SO̟ ̧Aq + 2H2O. 2 5 2 5 3 Oxide of nitrogen N,O, is acidic; oxide of sodium Na ̧O is 135 basic; NO, dissolves in water, and it may be proved that the solution contains the compound HNO; Na O dissolves in water, and on evaporation the compound NaOH is obtained. If a solution of NaOH is added to a solution of HNO until the liquid just ceases to affect the colour of litmus, and this solution is evaporated nearly to dryness, crystals separate having the composition NaÑO, (Na 23, N = 14, 0=16). An aqueous solution of this compound does not affect the colour of litmus and is without any of the marked charac = 136 137 teristics of an acid or an alkali. The interaction by which this compound has been produced is represented in an equation thus ; Na OHAq + HNO„Aq = Na NO,Aq + H2O. 2 NaOH is an alkali, HNO, an acid, and NaNO, a salt. When an alkali reacts with an acid to form a salt, the acid is said to be neutralised by the alkali, and the alkali to be neutralised by the acid. The salt does not exhibit the properties of either acid or alkali. The following equations represent interactions of alkalis and acids with production of salts: In each case the composition of the salt is, a metallic (electro-positive) element combined with the elements of the acid except hydrogen. This relation between the compositions of the acid and the salt is usually expressed by saying that the hydrogen of the acid is replaced (or displaced) by the metal of the alkali. But we have already learned that all basic oxides are not alkali-forming. Let us consider the interaction between a basic, but non-alkaline, oxide and an acid. The oxide of iron whose composition is expressed by the formula Fe2O2 (Fe = 56) is a basic oxide. This oxide is insoluble in water, but it dissolves in solutions of various acids, and on evaporating these solutions salts are obtained: thus, Fe 0, +3H SO,Aq= Fe,3SO,Aq + 3H2O. 3 Fe O+ 6HCl Aq= FeCl Aq + 3H 0. 3 3 In each case the relation between the composition of the salt and the acid may be expressed by saying that the metal of the basic oxide (iron) has replaced or displaced the hydrogen of the acid. An aqueous solution of the salt produced in each interaction does not affect the colour of litmus; it has not a sour or burning taste, and it does not corrode animal or vegetable membranes. Salts then are produced by the interactions between basic 138 oxides and aqueous solutions of acids. Salts are also generally produced when metals interact with aqueous solutions of acids. We have had examples of these reactions before (par. 124). The following may be added ;— Copper Copper nitrate Water and ni Zinc Nitric Zinc nitrate Water, and 2 Ba + 2HCIO,Aq = Ba2C10 ̧Aq + 2H Nitric 3Cu+8HNO,Aq=3Cu 2NO2Aq+4H2O+2NO tric oxide Zn+xHNO,Aq= Zn2NO, Aq + &c. nitric oxide and sometimes other compounds of nitrogen, The relation between the composition of the salt and the acid which interacted with a metal to produce the salt may in each case be expressed by saying that the hydrogen in one or more reacting weights of the acid has been replaced by the metal; in many reactions the displaced hydrogen is obtained, but in some cases further chemical change occurs, and water and compounds formed of the elements formerly combined in the acid are produced. Sometimes the whole of the hydrogen combined with other elements to form a reacting weight of an acid is not displaced by metal when a salt is produced; thus (1) KOHAq+ H2SO2Aq = KHSO ̧Aq + H2O. 2 139 140 Nevertheless each of the compounds in the third column (KHSO, &c.) is called a salt. So far as composition goes we may at present regard a salt as a compound of a metal with the elements of an acid except the whole or a part of the hydrogen of that acid; as regards mode of formation we may say, that a salt is one of the products of the interaction between an acid (in solution) and a metal, a basic oxide, or an alkali. The meaning of the term salt then evidently includes the meanings of the terms acid, metal, basic oxide, and alkali; and any one of these terms can be understood only by considering the meanings of them all. We already know the general characteristics of those elements which are called metals; we also know that most basic oxides are oxides of metals; and that alkalis are produced by the interaction of certain basic oxides with water. The meaning given above to the term salt implies that an acid is a compound of hydrogen. But all compounds of hydrogen are not acids. The characteristic of those compounds of hydrogen which are acids is that when they interact with metals, basic oxides, or alkalis, they exchange the whole or a part of their hydrogen for metal, and thereby form a salt, or salts. Alkalis also are compounds of hydrogen with oxygen and a metal. The composition of a reacting weight of the alkalis is represented by the formula MOH; where M = Li, Na, K, Rb, or Cs. These elements, Li, &c. are the most positive of all the elements. If the composition of acids is compared with the properties of the element or elements which combine with hydrogen to produce these acids, it is found that the non-hydrogenous constituents of acids, as a rule, are distinctly negative elements. This is shewn by comparing the compositions of the following well known acids with the arrangement of elements in electrical order given in par. 131. HÑO,, H2SO, HCN, H,C,O,, HCIO,, H,PO, HI, HC, HF, H.SIF, HBO, HSiO.. 2 But the relations between the compositions and properties of acids, alkalis, and salts cannot be properly elucidated until a later stage. The term base is frequently used to include basic oxides, alkaline hydroxides, and alkalis; as thus used it indicates a compound which interacts with an acid to produce a salt and water. Looking back for a moment we see that in attempting 141 to classify oxides we have been led to classify very many elements. We have divided these elements into electropositive and electro-negative. With the characters expressed by each of these terms we have connected several other physical, and some chemical, characteristics. The electro-positive elements, as a class, are metallic; their oxides are basic; the oxides of the more electro-positive elements interact with water to form compounds of the composition MOH which are alkalis. Many of these elements decompose water, or steam, with production of hydrogen and a metallic oxide. The electro-negative elements, as a class, are non-metallic; their oxides are generally acidic, that is they interact with water to produce acids. Some of these elements decompose steam with production of oxygen and a non-metallic hydride.* * For a fuller treatment of the subject of acids and salts s. Chap. XI. |