§ 113. FIFTH GROUP. OXIDE OF SILVER, SUBOXIDE OF MERCURY, OXIDE OF MERCURY, OXIDE OF LEAD, TEROXIDE OF BISMUTH, OXIDE OF COPPER, OXIDE OF CADMIUM. Properties of the group.-The sulphides corresponding to the oxides of this group are insoluble both in dilute acids and in alkaline sulphides.* The solutions of these oxides are therefore completely precipitated by hydrosulphuric acid, no matter whether their reaction be neutral, alkaline, or acid. For the sake of greater clearness and simplicity, we divide the oxides of this group into two divisions, and distinguish, 1. OXIDES PRECIPITABLE BY HYDROCHLORIC ACID, viz. : oxide of silver, suboxide of mercury, oxide of lead. 2. OXIDES NOT PRECIPITABLE BY HYDROCHLORIC ACID, viz.: oxide of mercury, oxide of copper, teroxide of bismuth, oxide of cadmium. Lead must be considered in both divisions, since the sparing solubility of its chloride might lead to confounding its oxide with suboxide of mercury and oxide of silver, without affording us, on the other hand, any means of effecting its perfect separation from the oxides of the second division. FIRST DIVISION OF THE FIFTH GROUP: OXIDES WHICH ARE PRECIPITATED BY HYDROCHLORIC ACID. Special Reactions. § 114. a. OXIDE OF SILVER (Ag O). 1. Metallic silver is white, very lustrous, moderately hard, highly malleable, ductile, rather difficultly fusible. It is not oxidized by ignition in the air. Nitric acid dissolves silver readily; the metal is insoluble in dilute sulphuric acid and in hydrochloric acid. 2. Oxide of silver is a grayish-brown powder; it is not altogether insoluble in water, and dissolves readily in dilute nitric acid. It forms no hydrate. It is decomposed by heat into metallic silver and oxygen gas. 3. The salts of oxide of silver are non-volatile and colorless; most of them acquire a black tint upon exposure to light. The soluble neutral salts do not alter vegetable colors, and are decomposed at a red heat. 4. Hydrosulphuric acid and sulphide of ammonium precipitate from solutions of salts of silver black SULPHIDE OF SILVER (Ag S), which is insoluble in dilute acids, alkalies, alkaline sulphides, and cyanide of potassium. Boiling nitric acid decomposes and dissolves this precipitate readily, with separation of sulphur. 5. Potassa and soda precipitate from solutions of salts of silver the oxide of this metal in the form of a LIGHT brown powder, which is insoluble in an excess of the precipitant, but dissolves readily in ammonia. * Consult, however, the paragraphs on oxide of copper and suboxide and oxide of mercury, as the latter remark applies only partially to them. 6. Ammonia, when added in very small quantity to neutral solutions of oxide of silver, throws down the oxide as a brown precipitate, which readily redissolves in an excess of ammonia. Acid solutions of silver are not precipitated. 7. Hydrochloric acid and soluble metallic chlorides produce in solutions of salts of silver a white, curdy precipitate of CHLORIDE OF SILVER (Ag Cl). In very dilute solutions these reagents impart simply a bluishwhite opalescent appearance to the fluid. By the action of light chloride of silver first acquires a violet tint, and ultimately turns black; it is insoluble in nitric acid, but dissolves readily in ammonia as ammonio-chloride of silver, from which double compound the chloride of silver is again separated by acids. Concentrated hydrochloric acid and concentrated solutions of chlorides of the alkali metals dissolve some chloride of silver, more particularly upon application of heat; but the dissolved chloride separates again upon dilution. Upon exposure to heat, chloride of silver fuses without decomposition, giving upon cooling a transparent horny mass. 8. If compounds of silver, mixed with carbonate of soda, are exposed on a charcoal support to the inner flame of the blowpipe, WHITE, BRILLIANT, DUCTILE METALLIC GLOBULES are obtained, unattended with incrustation of the charcoal. $ 115. b. SUBOXIDE OF MERCURY (Hg, 0). 1. Metallic mercury is grayish-white, lustrous, fluid at the common temit solidifies at 40°, and boils at 680° Fah. perature; It is insoluble in hydrochloric acid; in dilute cold nitric acid it dissolves to nitrate of suboxide, in more concentrated hot nitric acid to nitrate of oxide of mercury. 2. Suboxide of mercury is a black powder which is readily soluble in nitric acid, and is decomposed by the action of heat, the mercury volatilizing in the metallic state. It forms no hydrate. 3. The salts of suboxide of mercury volatilize upon ignition; most of them suffer decomposition in this process. Subchloride and subbromide of mercury volatilize unaltered. Most of the salts of suboxide of mercury are colorless. The soluble salts in the neutral state redden litmus paper. Nitrate of suboxide of mercury is decomposed by addition of much water into an insoluble basic and soluble acid salt. 4. Hydrosulphuric acid and sulphide of ammonium produce black precipitates of SUBSULPHIDE OF MERCURY (Hg, S), which are insoluble in dilute acids, sulphide of ammonium, and cyanide of potassium. Protosulphide of sodium dissolves this subsulphide to sulphide, with separation of metallic mercury. Bisulphide of sodium dissolves the subsulphide to sulphide, without separation of metallic mercury. Subsulphide of mercury is readily decomposed and dissolved by nitrohydrochloric acid, but not by boiling concentrated nitric acids. 5. Potassa and ammonia produce in solutions of salts of suboxide of mercury black precipitates, which are insoluble in an excess of the precipitants. The precipitates produced by potassa consist of SUBOXIDE OF MERCURY; whilst those produced by ammonia consist of a BASIC DOUBLE SALT OF SUBOXIDE OF MERCURY AND AMMONIA, e.g. (N H,, NO, + 2 Hg, O). 6. Hydrochloric acid and soluble metallic chlorides precipitate from solutions of salts of suboxide of mercury SUBCHLORIDE OF MERCURY (Hg, Cl), as a fine powder of dazzling whiteness. Cold hydrochloric acid and cold nitric acid fail to dissolve this precipitate; it dissolves, however, although very difficultly and slowly, upon long protracted boiling with these acids, being resolved by hydrochloric acid into chloride of mercury and metallic mercury, which separates; and converted by nitric acid into chloride of mercury and nitrate of oxide of mercury. Nitrohydrochloric acid and chlorine water dissolve the subchloride of mercury readily, converting it into chloride. Ammonia and potassa decompose the subchloride of mercury, and separate black suboxide from it. 7. If a drop of a neutral or slightly acid solution of suboxide of mercury is put on a clean and smooth surface of copper, and washed off after some time, the spot will afterwards, on being gently rubbed with cloth, paper, &c., appear WHITE and LUSTROUS like silver. The application of a gentle heat to the copper causes the metallic mercury precipitated on its surface to volatilize, and thus removes the silvering. 8. Protochloride of tin produces in solutions of suboxide of mercury a gray precipitate of METALLIC MERCURY, which may be united into globules by boiling the metallic deposit, after decanting the fluid, with hydrochloric acid. 9. If an intimate mixture of an anhydrous compound of mercury with anhydrous carbonate of soda is introduced into a drawn-out glass tube, and covered with a layer of carbonate of soda, and the tube is then heated before the blowpipe, the mercurial compound invariably undergoes decomposition, and metallic mercury separates, forming a coat of gray sublimate above the heated part of the tube. The minute particles of mercury may be united into larger globules by rubbing this coating with a glass rod. § 116. c. OXIDE OF LEAD (Pb O). 1. Metallic lead is bluish-gray; its surface recently cut exhibits a metallic lustre ; it is soft, malleable, readily fusible. Fused upon charcoal before the blowpipe, it forms a coating of yellow oxide on the charcoal. Hydrochloric acid and moderately concentrated sulphuric acid act upon it but little, even with the aid of heat; but dilute nitric acid dissolves it readily, more particularly on heating. 2. Oxide of lead is a yellow or reddish-yellow powder,* which upon exposure to a red heat fuses to a vitreous mass. Hydrated oxide of lead is white. Both the oxide and its hydrate dissolve readily in nitric and acetic acids. 3. The salts of oxide of lead are non-volatile; most of them are colorless; the neutral soluble salts redden litmus paper, and are decomposed at a red heat. 4. Hydrosulphuric acid and sulphide of ammonium produce in solutions of salts of lead black precipitates of SULPHIDE OF LEAD (Pb S), which are insoluble in cold dilute acids, in alkalies, alkaline sulphides, and cyanide of potassium. Sulphide of lead is decomposed by boiling concentrated nitric acid; the whole of the lead is first converted into nitrate of lead, and the greater part of the sulphur separates, whilst another portion is converted into sulphuric acid. The sulphuric acid thus formed decomposes a portion of the nitrate of lead, and forms with the liberated oxide sulphate of lead, which precipitates as a white powder; the deposit in the solution consists therefore of a mixture of sulphate of lead and sulphur. In solutions of salts of lead containing an excess of a concentrated mineral acid, hydrosulphuric acid produces a precipitate only after the addition of water or after neutralization of the free acid by an alkali. If a solution of lead is precipitated by hydrosulphuric acid, in presence of a large quantity of free hydrochloric acid, a red precipitate is formed, consisting of chloride and sulphide of lead, which, however, is converted by an excess of hydrosulphuric acid into black sulphide of lead. 5. Potassa and ammonia throw down BASIC SALTS OF LEAD in the form of white precipitates, which are insoluble in ammonia, and difficultly soluble in potassa. In solutions of acetate of lead ammonia does not immediately produce a precipitate, owing to the formation of a soluble triacetate of lead. 6. Carbonate of soda throws down from solutions of salts of lead a white precipitate of BASIC CARBONATE OF LEAD [e. g. 5 (Pb O, CO2) + Pb O, HO], which is insoluble in an excess of the precipitant and also in cyanide of potassium. 7. Hydrochloric acid and soluble chlorides produce in concentrated solutions of salts of lead heavy, white precipitates of CHLORIDE OF LEAD (Pb Cl), which are soluble in a large amount of water, especially upon application of heat. This chloride of lead is converted by ammonia into basic chloride of lead (Pb Cl, 3 Pb O + H O), which is also a white powder, but almost absolutely insoluble in water. In dilute nitric and hydrochloric acids, chloride of lead is more difficultly soluble than in water. 8. Sulphuric acid and sulphates produce in solutions of salts of lead white precipitates of SULPHATE OF LEAD (Pb O, SO1), which are nearly insoluble in water and dilute acids. From dilute solutions, especially from such as contain much free acid, the sulphate of lead precipitates only after some, frequently after a long time. It is advisable under all circumstances to add a considerable excess of dilute sulphuric acid, since sulphate of lead is more insoluble in this menstruum than in water. The separation of small quantities of sulphate of lead is best effected by evaporating, after the addition of the sulphuric acid, as far as practicable on the water-bath, and then treating the residue with water. Sulphate of lead is slightly soluble in concentrated nitric acid; it dissolves with difficulty in boiling concentrated hydrochloric acid, but more readily in solution of potassa. It dissolves also pretty readily in the solutions of some of the salts of ammonia, particularly in solution of acetate of ammonia; dilute sulphuric acid precipitates it again from these solutions. 9. Chromate of potassa produces in solutions of salt of lead a yellow precipitate of CHROMATE OF LEAD (Pb O, Cr O,), which is readily soluble in potassa, but difficultly so in dilute nitric acid. 10. If a mixture of a compound of lead with carbonate of soda is exposed on a charcoal support to the reducing flame of the blowpipe, soft, malleable METALLIC GLOBULES OF LEAD are readily produced, the charcoal becoming covered at the same time with a slight yellow incrustation of OXIDE OF LEAD. § 117. Recapitulation and remarks.—The metallic oxides of the first division of the fifth group are most distinctly characterized in their corresponding chlorides; since the different respective deportment of these chlorides with water and ammonia affords us a simple means both of detecting them and effecting their separation from one another. For if the precipitate containing the three metallic chlorides is boiled with a somewhat large quantity of water, or boiling water is repeatedly poured over it on the filter, the chloride of lead dissolves, whilst the chloride of silver and the subchloride of mercury remain undissolved. If these two chlorides are then treated with ammonia, the subchloride of mercury is converted into a black basic salt, insoluble in an excess of the ammonia; whilst the chloride of silver dissolves readily in that agent, and precipitates from this solution again upon addition of nitric acid. When operating upon small quantities, it is advisable first to expel the greater part of the ammonia by heat. In the aqueous solution of chloride of lead the metal may be readily detected by sulphuric acid. SECOND DIVISION OF THE FIFTH GROUP: OXIDES WHICH ARE NOT PRECIPITATED BY HYDROCHLORIC ACID. Special Reactions. a. OXIDE OF MERCURY (Hg O). ; 1. Oxide of mercury is generally crystalline, and has a bright red color, which upon reduction to powder changes to a pale yellowish-red ; the oxide precipitated from solutions of the nitrate of the oxide, or from solutions of the chloride, forms a yellow powder. Upon exposure to heat it transiently acquires a deeper tint; at a dull red heat it is resolved into metallic mercury and oxygen. Both the crystalline and non-crystalline oxide dissolve readily in hydrochloric and nitric acids. 2. The salts of oxide of mercury volatilize upon ignition; they suffer decomposition in this process; chloride, bromide, and iodide of mercury volatilize unaltered. Most of the salts of oxide of mercury are colorless. The soluble neutral salts redden litmus paper. The nitrate and sulphate of oxide of mercury are decomposed by water added in large quantity, into soluble acid and insoluble basic salts. 3. Addition of a very small quantity of hydrosulphuric acid or sulphide of ammonium produces in solutions of oxide of mercury, after shaking, a perfectly white precipitate. Addition of a somewhat larger quantity of these reagents causes the precipitate to acquire a yellow, orange, or brownish-red color, according to the less or greater proportion added; an excess of the precipitant produces a purely black precipitate of SULPHIDE OF MERCURY (Hg S). This progressive variation of color from white to black, which depends on the proportion of the hydrosulphuric acid or sulphide of ammonium added, distinguishes the oxide of mercury from all other bodies. The white precipitate which forms at first consists of a double compound of sulphide of mercury with the still undecomposed portion of the salt of oxide of mercury (in a solution of chloride of mercury, for instance, Hg C1+2 Hg S); the gradually in |