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in their physical and chemical properties; sodium is evidently closely allied to the even-series members; hydrogen must be considered apart from the other elements; copper, silver, and gold exhibit resemblances but also marked differences.

Odd-series ele-
COPPER.
SILVER.

GOLD.
ments
(hydrogen and so-

dium omitted)
Atomic weights

63.2
107.66

197
The molecular weights of these elements are unknown.
Sp. grs. (approx.)

8.8
10-5

19-5
Melting points
1050°
1000°

1200°
(approx.)
Sp. heats
095
*057

*0324
Atomic weights

7.2
10:3

10-1
spec. graus.
Appearance and Heavy, lustrous, red- White, very lustrous, Yellow-red, lustrous,
general physical dish, solid; very tena- hard, solid; very mal- rather soft, solid; ex-
properties. cious and ductile ; mal- leable, tenacious, and tremelê malleable and

leable; very good con- ductile; good conductor ductile; very tenacious;
ductor of heat and of heat and electricity. good conductor of heat
electricity.

Crystallises in regular and electricity.
Crystallises in regular octahedra.

octaliedra.
Occurrence and Metal occurs native; Metal occurs native; Metal occurs native;
preparation. also as sulphide &c., also as sulphide fre- generally alloyed with

frequently with similar quently with Cums, silver, and frequently compounds of silver, Sb2S3, &c.; AgČI also also with small quantiiron, &c. Fairly widely occurs. Most lead ores ties of copper and iron. distributed and in con- contain small quanti- Compounds of gold do siderable quantities. ties of silver.

not occur. Prepared by roasting Widely distributed, Widely distributed, Cuš in air till mixture generally in compara- generally in small of Ču0

and Cu2S is pro- tively small quantities. quantities. duced, then shutting off Prepared by heating Preparing by washing air and raising tempera- Ag28 with salt in air away gangue &c., some ture, when CugS+2CuO (Ag2S + 2NaCl + 40 times by mixing crushed =4Cu + S02

=Na2SO4+2 AgCl), auriferous quartz and then agitating with with mercury whereby iron (2AgCi + Fe a gold-amalgam is =FeCl2 +2Ag). formed, and then re.

moving Hg hy heating. General chemical Oxidised to Cup by Slowly oxidised by di- Not oxidised by direct properties. strongly heating in air. rect union with oxygen union with oxygen.

Combines directly with at very high tempera- Combines directly with
the halogens and with tures. Molten silver the halogens at mode-
sulphur, at moderately absorbs oxygen but the rately high tempera-
high temperatures. gas is evolved again as tures, but not with
Interacts with many the metal cools. sulphur.
acids to formi salts. Combines directly at Does not interact di-
Decomposes steam moderate temperatures rectly with many acids,
slowly at full red-heat with the halogens and insoluble in most acids;
giving CuO and H.
with sulphur

but a few salts are ob-
Interacts with many tained from the oxide.
acids to form salts. No reaction with steam.
Does not react with
steam.

438

General formulae and chemical characters of compounds. The only compounds which have been gasified and the vapourdensities of which have been determined are Cu,cl, and AgCl. The following formulae are not necessarily molecular.

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Copper forms two series of compounds, represented by the following ; (1) Cu,0, Cu,ci,, Cu,50; (2) Cuo, Cuci,, Cuso

CuSO Silver forms one series of compounds represented by Ag, o, Ag, Cl,, Ag,SO

Gold forms two series, represented by, (1) Au,0, Au Ci,, (2) Au Oy, Au, Cl, or Auci,

A hydride of copper, Cu, H, is known.

Oxides. M,O where M = Cu, Ag, Au; MO where M = Cu, Ag, Au; M,0, where M = Au only.

Sulphides. M S where M = Cu or Ag; MS where M = Cu; M,S, where M = Au.

Haloid compounds. M,cl, where M = Cu, Ag, Au; MCI, where M = Cu; MCI, where M = Au.

Salts. Most of the copper salts belong to the form CuX where X = SO, 2NO,, .CO, PO, 2010, &c.; a few are

,

&c known of the form Cu X. The definite silver salts all belong to the form Ag, X. Very few salts of gold are known; some seem to be similar in composition to the silver salts and to be represented by the formula Au,X, others belong to the form Au,3X.

The compositions of the compounds of copper silver and gold may be represented by the following general formulae ; X=0, S, Cl, Br,, I,, SO, 2003, 2NO, PO, &c.

M.X.
Cu,0, Cu,S, Cu,Cl2, Cu,I,, and a few unstable salts e.g. Cu,80z.
Ag,0, Ag, Cl2, Ag, 12, Ag, Br, &c., and all salts e.g. Ag,SO..
Au,0, Au,C1,, Au, lg, and a very few unstable salts e.g. Au 8,0z.

MX.
CuO, CuCl2, &c., and many well-marked salts.
Ago, no salts.
AuO, AuSO4

M,Xg.
Au,O2, Au,Clo, Aug(OH)o, and a few salts e.g. Au(NO3)3;HNO3.3H,0.
No Cu or Āg compounds.

The oxides MO are prepared; in the case of Ag,0 by 439 adding potash or soda to a solution of a salt of silver (e.g. 2 AgNO Aq + 2KOHAq = Ag,0 +2KNO, Aq+H,0); in the cases of Cu O and Au, O by reducing solutions of copper or gold salts in the presence of an alkali, CuSO, Aq may be reduced by grape sugar, and AuCl, Aq by sulphur dioxide.

Cuprous oxide Cu,o is fairly stable, but it is easily oxidised to CuO by heating in air; with acids it forms cupric salts, e.g. Cuso,; the cuprous salts, which correspond to Cuo,

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are not obtained from the oxide. Argentous oxide, Ag, O, is a stable compound which interacts with acids to form salts and water; e.g. Ag,0 + 2HNO, Aq=2 AgNO,Aq+H,0; it is decomposed at about 250° to silver and oxygen. This oxide is strongly basic, in some of its reactions it closely resembles the oxides of the alkali metals; an hydroxide of silver has not however been isolated. Aurous oxide, Au,0, is very easily decomposed; it rapidly absorbs oxygen with formation of auric oxide Au,0,: one or two salts corresponding to Au, are known, but they are not obtained from the oxide; the oxide is said to be soluble in cold water.

The oxides MO are prepared; CuO by adding an alkali to the solution of a cupric salt, and heating the hydrated oxide CuO.H.O which is precipitated (e.g. CuSO, Aq + 2KOHAq= CuO.HO+K SO, Aq; and CuO.H,O heated, even in contact with water, =Ču0+,O); Ago by passing ozonised oxygen over finely divided silver; AuO by heating hydrated auric oxide, Au OHO, to 160°

Cupric oxide CuO is basic, it interacts with acids to form a large series of salts CuX(X = SO,, &c.); this oxide is not decomposed by heat alone; most reducing agents remove the oxygen from heated Cuó. The oxide does not combine directly with water, but the hydrate CuO.H,0 is obtained, indirectly, as described above. Argentic oxide, or silver peroxide, Ago (or Ag,0,) is easily decomposed; heated to 110° it suddenly evolves oxygen and silver remains; this oxide interacts with acids to form argentous salts (e.g. Ag,SO,, Ag2NO, &c.) and oxygen, that is, it behaves as a peroxide.

Auro-auric oxide AuO (Au,0, is usually known as auric oxide) is easily decomposed ; at 173° it is separated into gold and oxygen; it does not appear to form any salts by reacting with acids.

Auric oxide, Au,o,, the only representative of the M.O, class of oxides, is prepared by adding an alkali to a solution of auric chloride Auci, (magnesia seems to be the best alkali to use), drying the precipitated Au,0,. H, O, and then carefully heating it to 100°. The oxide is easily decomposed by heat, or by exposure to light; it dissolves in nitric acid, and the salt Au(NO3)2.HNO,.38,0 can be obtained from this solution; it interacts with hydrochloric acid to form the compound AuCl.HCl, and with hydrobromic acid to form AuBr, HBr; these compounds are monobasic acids forming salts such as KAuCl, and KAuBr, Hydrated auric oxide, Au,0g. H,O,

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acts as an acid towards strong alkalis; it dissolves in concentrated KOHAq, and the salt potassium aurate KAO,.3H,0 may be obtained by evaporating in vacuo.

The sulphides MS (M=Cu or Ag) are obtained by the 440 direct union of the elements, or in the case of Ag, S by precipitating a solution of a silver salt by hydrogen sulphide. When hydrogen sulphide is passed into a solution of a salt of copper, cupric sulphide, Cus, is precipitated. The two sulphides of gold, AuS and Aus, seem to be obtained by the interaction of hydrogen sulphide with solution of auric chloride, AuCl,, under varying conditions.

The sulphides Cu,S, CuS, and Ag, S interact with acids to form salts and hydrogen sulphide, cupric salts being obtained from both sulphides of copper. Both sulphides of gold, Aus and Au Sy, dissolve in solutions of alkali sulphides to form thio-salts, the salts NaAuS and KAus, having been obtained ; these sulphides are therefore acidic.

The haloid compounds M,X, M=Cu, Ag, Au, are 441 obtained; in the case of copper and gold by reducing the compounds CuX,; in the case of silver by the interaction of haloid acids with silver oxide Ag,0. Cu, Cl, is formed by heating a solution of Cuci, in HCIAq with copper, or with sulphur dioxide, and pouring into water; Au Cl, is produced by dissolving gold in aqua regia, evaporating to dryness, and heating cautiously. By the interaction between AuCl and hydriodic acid, aurous iodide, Au, I,, is obtained

(2AuCl Aq + 6HIAq=Au, I, +41 + 6HC1Aq). Cuprous iodide is formed by reducing CuSO, Aq by sulphur dioxide and adding potassium iodide solution.

The haloid compounds of silver, AgCl, AgBr, AgI, are very stable compounds; Cu,Cl, has been gasified; Cul, is also a stable compound; Au,cl, is separated into its elements by heat or by the action of reducing agents.

The only representatives of the class of haloid compounds MX, are the cupric compounds. Cupric chloride, Cuci,, is obtained from the corresponding oxide, Cuo, by dissolving in HCIAq, and evaporating. This chloride combines with ammonia to form various compounds CuCl,.«NH,, it also combines with alkali chlorides to form double salts, e.g. CuCl,. 2KC1.2H,0. Various oxychlorides are also derived from CuCl,; their composition may be expressed by the general formula xCuCl,.yCuO.

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copper salts

Gold is the only element of the family which forms haloid compounds of the form MX, Auric chloride Aucl, is best obtained by adding water to aurous chloride. Solutions of this compound are very easily decomposed by reducing agents, or by light, with separation of gold. When gold is dissolved in aqua regia and the solution is evaporated, crystals of the monobasic chloro-auric acid, HAuCl, are obtained; bromoauric acid, HAuBr, has also been isolated ; and from each a

few definite salts have been formed, e.g. KAuCl, andKAuBr, 442 The salts of copper and silver are numerous and are very

definite compounds; only a few salts of gold have been isolated, and they seem easily to undergo change.

A few cuprous salts are known; one of the most definite is cuprous sulphite Cu, Sog, prepared by adding ammonium sulphite to a solution of copper sulphate, and then passing sulphur dioxide into the solution. The cupric salts, Cux, e.g. Cušo, Cu2NO,, CuCO3, Cu2C103, Cu,(PO),, &c. are usually obtained by dissolving moist copper oxide in the various acids. Many form double compounds especially with the salts of the alkali metals, e.g. CuSO :(NH.),80, 61,0. Several basic

are known, e.g. 3cůO. Ñ,0, 4Cu0. As,O, 8Cu0.280, Many copper salts combine with ammonia, e.g. CuSO,.4NÉ, Cu(NO3), 2NH, The salts of silver all belong to the form Ag, X, e.g. Ag,SO,, AgNO3, Ag,S,02, Ag, PO, Ag,CO, A few basic salts of silver are known chiefly derived from the weaker acids, e.g. 3Ag,0.200,; many silver salts combine with ammonia, e... AgNO3.3NH;, Ag,SO4NH,. Silver sulphate forms an alum Ag So,A1, (SO3)3. 24H,0. Only a few salts of gold have been prepared ; Au,8,0, is obtained combined with Na S,O, by mixing concentrated solutions of sodium thiosulphate and auric chloride and adding alcohol. Hydrated auric oxide Au Oz.H,0 dissolves in nitric acid, and from this solution the nitrates Au(NO3)2.HNO3.3H 0 and AuO. NO, are obtained. Sulphuric acid interacts with AuO.NO, to form the sulphate Au0.H.SO, and this on heating gives the sulphate Auso,

Many of the compounds and salts of gold are soluble in water ; e.g. AuCl2, Au, 0 (?), AuSoc. The oxides and haloid compounds, and the greater number of the salts, of silver are insoluble in water. The oxides of copper are insoluble in water; the cupric haloid compounds are soluble, the cuprous haloid compounds are insoluble, in water; many cupric salts are soluble in water.

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