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An aqueous solution of ammonia (NH) is markedly alkaline; it turns red litmus blue, combines with carbon dioxide to form ammonium carbonate, precipitates hydroxides of many metals from solutions of the salts of these metals, and neutralises acids to form salts. The salts thus produced are similar to, and generally identical or nearly identical in crystalline form with, the salts of the alkali metals. Compounds which react very similarly under similar conditions have usually similar compositions. If we represent the interactions between aqueous solutions of ammonia and potash with acids as follows we fail to trace the similarities of composition which we should expect to find :—
NH ̧Aq + HClAq= NH ̧ClAq: KOHAq+ HClAq = KClAq+H2O. 2NH.Aq+H2SO1Aq= N2H2SO,Aq: 2KOHAq+H SO1Aq= K2SO2Aq + 2H2O. NH ̧Aq+HNO_Aq = NH ̧ÑO2Aq:
KOHAq+ HNO2Aq = KNO2Aq + H2O.
The salts NH4Cl and KCl, N,H,SO, and K2SO,, NHNO and KNO, crystallise in the same forms. Identity, or very close similarity, of crystalline form usually accompanies similarity of chemical composition. The alkalis KOH, NaOH, &c. are metallic hydroxides. Let us assume that an aqueous solution of ammonia contains the hydroxide (NH)OH, that is a compound of the group of elements NH with oxygen and hydrogen, and that in the reactions of this compound with acids the group of elements NH, remains undecomposed, then we may represent the foregoing reactions thus,—
(NH,)OHAq + HClAq=(NH,)ClAq+H,O.
The formulae (NH)Cl and KCl, (NH),SO, and K,SO,, (NH)NO, and KNO,, (NH)OH and KOH, are evidently similar. In order to exhibit these similarities of composition we have assumed that a compound of one combining weight of nitrogen and 4 c.ws. of hydrogen takes the place of one c.w. of potassium in the reacting weights of the foregoing compounds; we have assumed that the compounds obtained by the interactions of an aqueous solution of ammonia (NH) with acids are compounds of negative elements (Cl, NO, SO, &c.) with the group of elements NH,, which group behaves in these compounds as if it were an elementary substance. The reasons
for making this assumption are the facts of similarity of properties between the compounds obtained by the interactions of the alkalis with acids and the compounds obtained by the interactions of an aqueous solution of ammonia with acids. We might represent the reaction between hydrocloric acid and ammonia solution in several ways; thus
(1) NH.H2OAq + HClAq = NH ̧.HClAq + H2O.
(2) NHAq+HCIAq=NH, HCỬA.
We choose (3) because this representation of the chemical change, more clearly than (1) or (2), suggests the analogies between this change and that which occurs when caustic potash solution interacts with hydrochloric acid.
A study of the properties of a class of compounds has thus led to a special view of the composition of these compounds.
Such a hypothetical group of elements as NH, is called a 211 compound radicle; this especial compound radicle NH is called ammonium. The salts of ammonium are analogous to the salts of potassium, sodium, lithium, rubidium, and caesium. Therefore, it may be argued, if ammonium were isolated it would probably be similar in properties to the alkali metals; and if ammonium hydroxide (NH)OH were isolated its properties would be similar to those of the hydroxides of the alkali metals.
Neither ammonium nor ammonium hydroxide has been isolated. A solid compound is known having the composition NCH,,O, this compound resembles potassium hydroxide; it dissolves in water forming a strongly alkaline solution which reacts with acids to form salts, &c. The methods whereby this compound is formed indicate that it is a hydroxide of the radicle N(CH), the formula N(CH),OH represents the compound in question as derived from (NH)OH by replacing H, by (CH). We have here carried the conception of compound radicle a step further. Facts seem to warrant the supposition that the four combining weights of hydrogen in the group NH, may be replaced by the radicle CH, and that the more complex group-N(CH),—so obtained may react like an element.
The conception of compound radicles will be more fully 212 developed later (s. chap. XVII.). Meanwhile it is important to notice that what we have learned emphasises the importance of regarding the reacting weight of a compound as a definite
mass of that compound composed of definite numbers of smaller parts (combining weights) of two or more elements, which smaller parts are arranged in some definite way relatively to each other. The reactions of ammonium compounds, for instance, almost oblige us to think of the reacting weights of these compounds as each composed of one combining weight of nitrogen closely united, in some way, with 4 c.ws. of hydrogen, and this group of combining weights as less closely united with the other elements which form a part of the reacting weight of the compound.
Let us return to the consideration of the properties of the hydrides MH. These compounds may all be oxidised : phosphorus hydride is very easily changed to phosphorus pentoxide (PO) and water by mixing with oxygen and heating; arsenic and antimony hydrides are oxidised to oxide of arsenic or antimony (MO) and water, by burning in contact with a large quantity of oxygen; ammonia is oxidised with difficulty, it is necessary to mix ammonia with a large quantity of oxygen and raise the temperature considerably, the products are water, nitrogen oxides (especially NO and N,O,), and nitrogen.
214 The oxides of the nitrogen group of elements are numerous; the following table presents the compositions of the best studied of these oxides.
M=N, P, Sb, or Bi.
When any one of the elements, except nitrogen, is heated in oxygen the oxide M2O, is formed; in the case of phosphorus, PO, and in the case of antimony, Sb,O,, is also produced. Nitrogen and oxygen combine when electric sparks are passed through a mixture of the gases; NO, and NO are produced. The oxides M2O, are usually produced from MO, by an interaction between M,O, and some compound (e.g. nitric acid), or compounds (e.g. caustic potash solution and chlorine), from which oxygen is produced. The lower oxides M.O, and M.O are formed by reducing the higher oxides; the methods of reduction employed are very indirect. The oxides M2O, are changed to M,O,, by the direct action of oxygen. The oxides M,O,, when MN or Sb, are changed to M2O by the direct action of oxygen; when MP the oxide is easily changed to M,O, by the direct action of oxygen; when M = As
or Bi the oxides are unchanged by heating in oxygen. oxides M,O,, except P2O,, are decomposed by heat, giving off oxygen, and forming either M2O, (M = N or Sb), or M2O (M = As or Bi).
Nitrous oxide (NO) dissolves in water without forming an acid; but as the oxide can be obtained by heating an aqueous solution of hyponitrous acid (HNOAq), the oxide may be regarded as the anhydride of this acid.
The oxides M2O, cannot be classed either as distinctly acidic or basic.
The oxides M,O,, except Bi,O,, are acidic; NO, and P,O, dissolve in water to form nitrous acid HNO,, and phosphorous acid H.PO,, respectively (N ̧O2+ H2O+ Aq = 2HNO,Aq; P ̧0 ̧+ 3H ̧Ỏ + Åq = 2H,PO2Aq); arsenious acid (?H,ASO) has not been isolated, but salts, e.g. KASO,, are obtained by the reaction of As,, with alkalis and basic oxides; antimonious acid H.SbO, is known, although it is not obtained by the reaction of water with the oxide Sb.O... The oxide Bi̟ O̟ is basic; it interacts with acids to form salts and water, thus Bi2O2+6HNO2Aq = 2Bi3NO2Aq + 3H2O. Besides being acidic towards alkalis and the more distinctly basic oxides, the oxides As2O, and SbO, are basic towards many acids; thus each interacts with hydrochloric acid to form a chloride and water; M2O2+6HClAq = 2MCl2Aq + 3H2O. These oxides also appear to interact with concentrated sulphuric acid to form sulphates M,3SO,, but there is some doubt as to the compositions of the products of these reactions.
The oxides M2O, are distinctly acidic, except Bi,O,. When M = N or P, the oxides dissolve in water to form the corresponding acids; the acids corresponding to the oxides when M = As or Sb are not obtained directly from these oxides by interaction with water. Bismuthic oxide, or bismuth pentoxide, (Bi̟O) interacts with acids to form the same salts as are produced when Bi,O, interacts, and oxygen is simultaneously evolved; bismuthic oxide is therefore a peroxide. From what we have learned concerning the properties of oxides, and from considering the properties of the highest oxides of the elements classed with bismuth, we might expect bismuthic oxide to exhibit some acidic functions. No salts are obtained by interactions between this oxide and alkalis; but when the oxide in question is prepared by passing chlorine into very concentrated potash solution holding bismuthous oxide (BiO) in suspension, the properties of the substances obtained render
it very probable that unstable compounds of bismuthic and potassium oxides are formed at certain stages of the preparation. The composition of these unstable salts may be expressed by the formula xBiO..yK,0; x and y probably vary according to the relative masses of Bi2O, and KOH used in the preparation of Bi,O,, and according to the temperature, &c. If bismuthic oxide exhibits any acidic functions they are certainly extremely feeble.
The following table presents the compositions of the more important oxyacids of the members of the nitrogen group; to each oxyacid there generally corresponds a certain oxide; that is, the oxide is obtained from the acid by removing hydrogen and oxygen (generally by heating the acid), or the acid is produced by the interaction of water and the oxide.
Oxyacid. salts known of forms MASO, and MASO2 (M=K, &c.)
loxyacid. HSbO,. H2SьO. HSb2O.
It will be noticed that the pentoxides of phosphorus, arsenic, and antimony, are represented as being each the anhydride of three acids.
Phosphorus pentoxide interacts with water to produce one or other of the three acids according to the relative masses of water and oxide used, and the temperature. The following equations represent the reactions ;
(1) P.0, + H2O (cold) = 2HPO.;
(2) PO+ 2H2O (cold) = H,P,O,;
(3) P2O + xH2O (warm) = 2H,PO ̧ + (x − 3) H ̧O.
* The symbol Aq is here used to signify that the acids after which it is placed are known only in aqueous solutions.