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With these physical characters are associated certain distinct chemical characters; e.g. nature of the products of the reactions of each class of elements with water. Of these chemical characters we shall learn more as we proceed, meanwhile we have to look especially to the chemical properties of the oxides of each class. Looking at the matter broadly we may assert that the oxides of the non-metallic elements are acidic, and the oxides of the metallic elements are basic. Further we may assert that most of the non-metals combine with hydrogen to form hydrides, but that very few hydrides of the metals are known.
We have now associated certain properties of several elements with one definite chemical characteristic of the oxides of these elements.
Non-metallic elements are electro-negative as regards metallic elements; their oxides are generally acidic; most of them form hydrides. Metallic elements are electro-positive as regards non-metallic elements; their oxides are generally basic; few of them form hydrides.
ACIDS AND SALTS.
THE sketch of the chemical properties of oxygen and hydrogen in Ch. viii. has shewn that, in order to learn anything of these properties we have been obliged to examine the properties of many of the compounds of these elements, and to do this we have found it necessary to study the properties of very many other elements. Our attempt to classify oxygen and hydrogen has taught us, more fully than before, that the chemical properties of this or that definite kind of matter are the properties exhibited in the interactions of the specified kind of matter with other kinds, both elements and compounds.
We have also learned that a classification of elements carries with it a classification of compounds, and vice versa; and we have found that the method of chemical classification is based on the study both of the composition and the properties of the kinds of matter to be classified.
The study of the chemical properties of hydrogen and oxygen led to a consideration of the meaning of the terms acidic and basic oxides, and this brought with it a consideration of the properties of acids and salts. These classes of compounds are of great importance in the classification of elements and compounds. We must now consider them somewhat more fully.
Sulphur trioxide (SO2) is a typical acidic oxide ; potassium oxide (K,O) is a typical basic acide. Let each be dissolved in water and each solution be evaporated; the first solution, until it becomes thick ; the second, until all the water is removed and a solid remains. Then let the thick oily liquid obtained by evaporating the first solution be cooled to - 10° or so; crystals separate ; let these be collected with suitable precautions and analysed; their composition is expressed by the formula
H.SO, (S = 32, 0 = 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, 0 = 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 H.So, is an acid, the compound KOH is an alkali.
A measured portion of the aqueous solution of H,SO, 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 K SO, 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) H SO (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+H SO, Aq=K,SO, Aq + 2H,0. Oxide of nitrogen N, O, is acidic ; oxide of sodium NaO is 135 basic; N, O, dissolves in water, and it may be proved that the solution contains the compound HNO3, NaOdissolves 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 NaNO3 (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
teristics of an acid or an alkali. The interaction by which this compound has been produced is represented in an equation
NaOHAq + HNO,Aq = Na NO,Aq+H,0.
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 :
HC1Aq+ NaOHAq= NaClAq+H_0.
HClO4q+ ROOHAT = RLCIỎ,Aq+ HO.
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 Fe,0, (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 380, Aq + 3H,O.
Fe, 0, +6H,C, Aq=Fe,6CAq+36,0.
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 ;
Other products Metal Solution of acid
of reaction Iron Sulphuric
Iron sulphate Hydrogen
Iron nitrate Water and ni. 2Fe+8HNO, Aq=Fe, 6NO, Aq+4H,O+2NO tric oxide Iron Hydrochloric
Îron chloride Hydrogen
Sodium nitrite Hydrogen
Sodium oxalate Hydrogen
Ba + 2HC10, Aq = Ba2C10 Aq + 2H
Copper nitrate Waterandni-
according to temperature and concentration of acid used. 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+H,SO, Aq = KHSO, Aq+H,.