412 PEINCIPLES OF CHEMISTRY 



The commonest normal salt is blue vitriol i.e. the normal cupric 

 sulphate. It generally contains five molecules of water of crystallisa- 

 tion, CuS0 4J 5H 2 O. It forms the product of the action of strong sul- 

 phuric acid on copper, sulphurous anhydride being evolved. The same 

 salt is obtained in practice by carefully roasting sulphuretted ores of 

 copper, and also by the action of water holding oxygen in solution on 

 them : CuS -fO 4 = CuSO 4 . This salt forms a by-product, obtained in 

 gold refineries, when the silver is precipitated from the sulphuric acid 

 solution by means of copper. It is also obtained by pouring dilute 

 sulphuric acid over sheet copper in the presence of air, or by heating 

 cupric oxide or carbonate in sulphuric acid. The crystals of this salt 

 belong to the triclinic system, have a specific gravity of 2-19, are of a 

 beautiful blue colour, and give a solution of the same colour. 100 

 parts of water at dissolve 15, at 25 23, and at 100 about 45 parts 

 of cupric sulphate, CuS0 4 . 10 bi8 At 100 this salt loses a portion of its 



is the same as malachite, ep. gr. 8'5) : 2CuS0 4 + 2Na 2 CO s + H 2 = CuCO 5 ,CuH 2 3 

 + 2Na 2 SO 4 + C0 2 . If the resultant blue precipitate be heated in the liquid, it loses water 

 and is transformed into a granular green mass of the composition Cu 2 CO 4 i.e. into a 

 compound of the normal salt with anhydrous cupric oxide. This salt of the oxide corre- 

 Bponds with orhocarbonic acid, C(OH) 4 = CH 4 O 4 , where 4H is replaced by 2Cu. On 

 further boiling this salt loses a portion of the carbonic acid, forming black cupric oxide, 

 BO unstable is the compound of copper with carbonic anhydride. Another basic salt which 

 occurs in nature, 2CuCO 3 ,CuH 2 O 2 , is known as azurite, or blue carbonate of copper ; it 

 also loses carbonic acid when boiled with water. On mixing a solution of cupric sulphate 

 with sodium sesquicarbonate no precipitate is at first obtained, but after boiling a pre- 

 cipitate is formed having the composition of malachite. Debray obtained artificial 

 azurite by heating cupric nitrate with chalk. 



10 bis Although sulphate of copper usually crystallises with 5H 2 O, that is, differently 

 to the sulphates of Mg, Fe, and Mn, it is nevertheless perfectly isomorphous with them, 

 as is seen not only in the fact that it gives isomorphous mixtures with them, containing 

 a similar amount of water of crystallisation, but also in the ease with which it forms, 

 like all bases analogous to MgO, double salts, R 2 Cu(SO 4 ). 3 ,6H 2 O, where R=^K, Rb, Cs, 

 of the monoclinic system. 



Salts of this kind, like CuCl 2 ,2KCl,2H 2 O,PtK 2 Cy 4 , &., present a composition 

 CuX 2 if tho representation of double salts given in Chapter XXIII., Note 11, be 

 admitted, because they, like Cu(HO)o, contain Cu(X 2 K) 2 , where X 2 =SO 4 , i.e. the 

 resuhie of sulphuric acid, which combines with H 2 , and is therefore able to replace 

 the H 2 by X 2 or O. A detailed study of the crystalline forms of these salts, made by 

 Tutton (1893) (see Chapter XIII., Note 1), showed : (1) that 22 investigated salts of the 

 composition R 2 M(SO 4 ),6H 2 O, where R = K, Rb, Cs, and M = Mg, Zn, Cd, Mn, Fe, Co, Ni, 

 Cu, present a complete crystallographic resemblance ; (2) that in all respects the Rb 

 salts present a transition between the K and Cs salts ; (3) that the Cs salts form 

 crystals most easily, and the K salts the most difficultly, and that for the K salts of Cd 

 and Mn it was even impossible to obtain well-formed crystals ; (4) that notwithstanding 

 the closeness of their angles, the general appearance (habit) of the potassium compound 

 differs very clearly from the Cs salts, while the Rb salts present a distinct transition in 

 this respect ; (5) that the angle of the inclination of one of the axes to the plane of the two 

 Other axes showed that in the K salts (angle from 75 to 75 38') the inclination is least, 

 in the Cs salts (from 72 62' to 73 50') greatest, and in the Rb salts (from 73 57' to 

 74 42') intermediate between the two ; the replacement of Mg. . . Cu produces but a 



