SMITH'S INTERMEDIATE CHEMISTRY 



,n be filed, turned, or polished like a mirror, and is better for 

 r v many purposes than the pure metal. 



On account of its great chemical activity (p. 54), aluminium 

 displaces many other metals (e.g. iron, manganese, and chromium) 

 from combination. Thus powdered aluminium and oxide of 

 iron when mixed (tJyaBiite) in a crucible, and started by a burning 

 magnesium ribbon/ interact with great violence : 



2A1 + Fe 2 3 -> A1 2 O 3 + 2Fe. 



A temperature of 3000 to 3500 is reached, the molten iron (m.-p. 

 1500) collects at the bottom, and the molten aluminium oxide 

 (m.-p. 2050) floats to the top. Steel rails are welded together, 

 and large objects of steel like broken propeller shafts are mended, 

 by enclosing a mass of thermite round the joint and firing it. 



Aluminium Hydroxide A1(OH) 3 . The hydroxide is precipi- 

 tated when ammonium hydroxide, or other alkaline hydroxide, 

 is added to a solution of a salt of aluminium: 



A1 2 (SO 



6NH 4 OH -> 2A1(OH) 3 



3(NH 4 ) 2 S0 4 (dslvd.). 



It tends to remain in colloidal suspension^ CD. 441), and forms a 

 white gelatinous precipitate. It is both weakly basic and feebly 

 ic in chemical properties. In acids it dissolves forming salts 

 of aluminium, such as the chloride A1C1 3 or sulphate A1 2 (S0 4 ) 3 . 

 Solutions of these salts in water give an acid reaction, owing to 

 hydrolysis (p. 369). 



Aluminium hydroxide dissolves also in sodium hydroxide solu- 

 tion, to form sodium aluminate NasA10 3 : 



3NaOH + HaAlOs -> NagAlOa + 3H 2 0. 



The aluminates are hydrolyzed by water, and their solutions 

 have an alkaline reaction. 



Aluminium hydroxide, precipitated from the sulphate, is used 

 in sizing paper (to fill the capillaries and pores), in purifying 

 water (see p. 470), and as a mordant_(see p. 475) in dyeing. Deli- 



