1. Process: common aluminum salt + ammonia

　　Al2(SO4)3+6NH3·H2O=2Al(OH)3↓+3(NH4)2SO4

　　Al3++3NH3·H2O=Al(OH)3↓+3NH4+

　　2. Nature

　　Al(OH)3 is a white gum that is almost insoluble in water and can condense suspended solids in water. Adsorption of pigments can purify water. Why not use a strong alkali solution such as NaOH when using Al(OH)3?

　　Al(OH)3 is soluble in both acid and alkali, forming salt and water.

　　Al(OH)3+3HCl=Al(OH)3+3H2O

　　Al(OH)3+3H+=Al3++3H2O

　　Al(OH)3+NaOH=NaAlO2+2H2O

　　Al(OH)3+OH-=AlO2-+2H2O

　　Experiment 1: Add NaOH solution to AlCl3 solution to excess

　　Al(OH)3+OH-=AlO2-+2H2O

　　AL(OH)3+NaOH=NaAlO2+2H2O

　　↓

　　Al3++3OH-=Al(OH)3↓

　　Al(OH)3+OH-=AlO2-+2H2O

　　Phenomenon: first produce a white precipitate, then the precipitate gradually dissolves

　　Experiment 2: Add AlCl3 solution to the NaOH solution to the excess

　　AlCl3+4NaOH=NaAlO2+3NaCl+2H2O

　　3NaAlO2+AlCl3+6H2O=4Al(OH)3↓+3NaCl

　　↓

　　Al3++4OH-=AlO2-+2H2O

　　Al3++3AlO2-+6H2O=4Al(OH)3↓

　　Phenomenon: no precipitation at first, white precipitate after a while and insoluble