Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/541

AND GOLD.] is dissolved by ammonia to a deep blue solution ; the production of deep blue-coloured liquids on dissolution in ammonia is, in fact, characteristic of copper compounds. Cupric oxide, CuO, dissolves in acids, forming cupric salts, which are white when anhydrous, but furnish green or blue solutions ; they are mostly soluble. Cupric sulphate crystallizes with 5 molecules of water, but it forms a double sulphate with potassium sulphate, CuK 2 (SO 4 ) 2 , 6H 2 O, isomorphous with the corresponding zinc and magnesium salts. Basic cupric salts are readily produced by digesting the normal salts with cupric hydroxide. By igniting cupric oxide with metallic copper it is con verted into red cuprous oxide, Cu 2 O ; this oxide may also be prepared by heating a solution of a cupric salt with an easily oxidizable substance, such as grape sugar, and an alkali. Most acids decompose cuprous oxide, forming a cuprio salt and separating metallic copper ; but it is con verted into cuprous chloride by hydrochloric acid. A number of cuprous salts exist, but they are extremely unstable, and mostly absorb oxygen readily, becoming converted into cupric salts. Both oxides of copper are easily reduced by ignition with hydrogen or charcoal. A black hydrated cupric sulphide is precipitated by hydrogen sulphide even from strongly acid solutions of cupric salts. Cuprous hydride, Cu 2 H 2 , is deposited as a yellow precipitate when a solution of cupric sulphate mixed with hypophosphorous acid is heated to about 60 C.; it soon turns brown, and when heated to 70 C. suddenly decom poses, with evolution of hydrogen ; hydrochloric acid converts it into cuprous chloride : CuH 2HC1 = Cu 2 Cl 2 -f 2H 2 . Silver is a white metal, inferior in malleablity only to gold among the heavy metals ; it is harder than gold, and softer than copper. It is the best-known conductor of heat and electricity. It may be distilled by the aid of the oxy-hydrogen flame. It is not oxidized by ordinary oxygen, and is incapable of decomposing water, but like mercury it is oxidized by ozone. Moderately diluted nitric acid dissolves it readily, and it is dissolved by heated concen trated sulphuric acid; it also resembles copper and mercury in its behaviour with hydrochloric acid, and mercury in its behaviour with hydriodic acid. It unites with the halogens at ordinary temperatures, and, like mercury, readily combines with sulphur, and is precipitated by hydrogen sulphide from strongly acid solutions of its salts as black sulphide, Ag. 2 S. Silver forms two chlorides, argentic chloride, AgCl, which is white, and insoluble in w r ater, but readily soluble in ammonia, and a black argentous chloride, the composi tion of which is not known with certainty, but is probably Ag 2 Cl 2 . Corresponding bromides exist, but only the one iodide, AgT, is known. Argentic chloride becomes violet- coloured on exposure to light, apparently owing to the formation of the lower chloride ; the iodide does not alter in appearance, but acquires the property of attracting metal lic silver, and hence its employment in photography. The subchloride is not changed by nitric acid, but is converted by ammonia into metallic silver and argentic chloride. A brown argentous hydroxide, Ag (OH), is precipitated from solutions of argentic salts on the addition of an alkali ; it is slightly soluble in water and its solution is faintly alkaline ; it loses W 7 ater at a temperature above 60 C., and is converted into argentic oxide, AgO. Argentic oxide is a brown powder ; it gives off a certain amount of oxygen, even at 100 C., and also when exposed to sunshine; it neutralizes the strongest acids, forming argentic salts such as argentic nitrate, AgXO 3 , Argentic sulphate, Ag 2 SO 4 , is isomorphous with anhydrous sodium sulphate, and it also 529 forms an alum, Ag 2 Al 2 (S0 4 ) 4 , 24H 2 O, which, however, is extremely unstable. Silver also forms a suboxide probably of the composition Ag 4 O 2 , and a peroxide of which the composition is uncertain, but probably is expressed by the formula Ag 9 2 . Both are extremely unstable ; the former is decomposed" by ammonia into metallic silver and argentic oxide, which dissolves, and acids act upon it in a similar manner. Argentic carbonate is insoluble in water, and decomposes at 200 C. into the oxide and carbon dioxide. Metallic silver is slowly precipitated from solutions of its salts by hydrogen at ordinary temperatures, so that it is a less positive element than hydrogen. Gold is a metal of rich yellow colour nearly as soft as lead when in a pure state. It exhibits no tendency to combine directly with oxygen, and is not dissolved by any single acid, except selenic acid, but it readily combines with the halogens, and therefore dissolves in a mixture of nitric and hydrochloric acids, which liberates chlorine (p. 466). Gold is reduced from its compounds with extreme readiness, but varies in properties according to the nature of the compound from which it is separated ; in fact, it appears to exist in several allotropic modifications. Thus, according to Thomsen, when a solution of auric chloride is reduced by sulphurous acid, the gold separates as a light- coloured powder which readily cakes together, but from a solution of auric bromide it separates as an extremely fine dark-coloured powder which exhibits no tendency to agglomerate ; a third modification, also finely pulverulent, but lustrous, is obtained by reducing aurous bromide or iodide with sulphurous acid. Thomsen finds that the con version of the second (Aua) and third (Au/rf) of these modifications into that obtained from auric chloride (Au) would involve development of heat, and of different amounts in the two cases; thus : Auj8 = Au + 4700 units of heat. Aua =Au + 3210 By the action of chlorine, reduced gold is converted into the chloride, Au 2 Cl 4 ; this is decomposed by water into aurous chloride, Au 2 Cl 2 , and auric chloride, AuCl 3 . Auric chloride is soluble in water, but aurous chloride is insoluble; in contact with water, especially on warming, the latter decomposes into metallic gold and auric chloride. Auric chloride is readily converted into aurous chloride when heated to 185 C. The gold bromides may be obtained in a similar manner, and exhibit similar properties, but are less stable. Aurous and auric iodides, Au 2 I 2 and AuI 3 , are both insoluble, and still less stable. The auric compounds combine with the haloid acids, and with the haloid com pounds of most other metals, forming crystalline so-called double salts ; these double salts, however, may be regarded as salts of the acids formed by the union of the haloid acids with the auric halogen compounds ; the double chloride of gold and potassium, for example, KCl.AuCl 3 , as the potassium salt of the acid HAuCl 4 . On treating aurous chloride with a solution of potassium hydroxide, a dark-green substance is obtained, which is probably the corresponding hydroxide ; it decomposes spontaneously into metallic gold and auric hydroxide. Auric hydroxide, Au(OH) 3 , is precipitated by alkalies from a solu tion of auric chloride as a dark-brown powder, which is quickly reduced on exposure to light. A third gold hydrox ide, derived from the oxide Au 2 O 2 , appears also to exist. The hydroxides of gold exhibit very feeble basic pro perties ; in fact, the salts formed on dissolving auric hydroxide in nitric and sulphuric acids are so unstable that they have not been isolated. Auric hydroxide, however, possesses marked acid properties, and is dissolved by a solution of potassium hydroxide, potassium aurate, a

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