ALUMINUM. 287 



The official alum, (i/iimen, is the potassium alum, KA1(SO 4 ) 2 .12H 2 O 

 = 471.02, a white salt crystallizing in large octahedrons, soluble in 

 10 parts of cold and 0.3 part of boiling water; this solution has an 

 acid reaction and a sweetish astringent taste. 



Alum is manufactured on a large scale by decomposing certain 

 kinds of clay (aluminum silicates) by sulphuric acid, when aluminum 

 sulphate is formed, to the solution of which potassium or ammonium 

 sulphate is added, when, on evaporation, potassium or ammonium 

 alum crystallizes. 



Dried alum ; Alumen exsiccatum, KA1(SO 4 ) 2 = 256.46 (Burnt 

 alum). This is common alum, from which the water of crystallization 

 has been expelled by heat. It is a white powder, dissolving very 

 slowly in cold, but quickly in boiling water. 



Aluminum hydroxide, Alumini hydroxidum, A1(OH) 3 = 77.54. 

 Obtained by adding ammonia water or solution of sodium carbonate 

 to solution of alum, when aluminum hydroxide is precipitated in the 

 form of a highly gelatinous substance, which, after being well washed, 

 is dried at a temperature not exceeding 40 C. (104 F.). 



2KA1(SO 4 ) 2 + 6NH 4 OH = K 2 SO 4 + 3(NH 4 ) 2 SO 4 + 2A1(OH) 3 ; 

 2KA1(SO 4 ) 2 + 3Na 2 CO 3 + 3H 2 O = K 2 SO 4 + 3Na 2 SO 4 + 3CO 2 + 2A1(OH) S . 



When aluminum hydroxide is heated, water is expelled and the 

 oxide is left, which is often termed alumina. 



The usual decomposition between a soluble carbonate and any soluble salt 

 (provided decomposition takes place at all) is the formation of an insoluble 

 carbonate ; according to this rule, the addition of a soluble carbonate to alum 

 should produce aluminum carbonate. The basic properties of aluminum 

 oxide, however, are so weak that it is not capable of uniting with so weak an 

 acid as carbonic acid, and it is for this reason that the decomposition takes 

 place as shown by the above formula, with liberation of carbon dioxide, 

 while the hydroxide is formed. (Other metals, the oxides of which have weak 

 basic properties, show similar reactions, as, for instance, chromium, and iron in 

 the ferric salts.) 



The weak basic properties of aluminum are shown also by the fact that alu- 

 minum sulphate, chloride, and nitrate, and even alum itself, have an acid 

 reaction, while the corresponding salts of the alkalies or alkaline earths are 

 neutral. 



Aluminum salts in solution give the ion Al*'% which forms insoluble 

 compounds with hydroxyl ion (OH) 7 , carbonate ion COg", phosphate ion 

 PO/ X/ , sulphide ion S /x , etc. The carbonate and sulphide are hydrolyzed 

 with elimination of CO 2 and H 2 S respectively. Aluminum hydroxide has 

 such weak basic properties that it actually shows an acid character toward the 



