76 CHEMISTRY FOR AGRICULTURAL STUDENTS 



left, in which, it is to be expected, the salt produced by the 

 action of sulphuric acid on oxide of zinc is dissolved. 



Zn + H20 = ZnO + H2; but Zn + Hp.SOs^ZnO.SOg + Hg^ 



Dissolve zinc in dilute sulphuric acid, and, when action 

 ceases, pour off the clear liquid from the excess of zinc and 

 evaporate on a water bath until, on cooling, crystals are ob- 

 tained. When quite cold, drain the crystals and purify them 

 by recrystallisation, i.e. by redissolving in pure water and eva- 

 porating till crystals are again obtained. Dry by pressing 

 between folds of filter paper and preserve. Note that the 

 crystals are isomorphous (of the " same form ") with those of 

 Epsom salt, showing that even in respect to the properties of 

 their compounds these metals belong to the same class. 



Only metals which are able to attack water, such as sodium, calcium, 

 zinc, and iron, are able to attack dilute sulphuric acid. Copper and mer- 

 cury are without action on the dilute acid, but their sulphates can, of 

 course, be obtained by the action of sulphuric acid on their oxides. Nearly 

 all the common metals are oxidised when heated with strong sulphuric acid, 

 sulphurous anhydride and water being produced ; the oxides formed react 

 with more of the sulphuric acid forming the corresponding sulphates. It 

 should be noted that the compound that acts as an oxidising agent is itself 

 reduced ; the substance that acts as a reducing agent is itself oxidised. 



While the formula equation given above represents the chemical change 

 in a simple way, it must not be supposed that the arrangement of the 

 elements in the compounds is in any way represented. The change in fact 

 might be represented more correctly thus : — 



Zn + H2SO4 = ZnS04 + Hg, 



in which the group SO4 is regarded as combined with hydrogen in the acid 

 and zinc in the salt, the acid being sulphate of hydrogen and the salt sul- 

 phate of zinc. There are many reasons for preferring this nomenclature 

 for metallic salts. To take the simplest reason, it is inconvenient to speak 

 of sulphate of oxide of zinc, sulphate of oxide of iron, and sulphate of 

 oxide of copper. It is usual to call these substances — 



Sulphate of zinc (white vitriol) . . . . ZnSO^.yH.jO 



Sulphate of iron (green vitriol) . . . . FeS04.7li20 



Sulphate of copper (blue vitriol) . . . CUSO4.5H2O 



and, to be consistent, to speak of the sulphates of sodium, potassium, mag- 



