White zinc hydroxide can be precipitated by adding an appropriate amount of a base to a solution of a soluble salt of Zn 2+ :
Zn 2+ +2NaOH=Zn ( OH ) 2 â†“ +2Na +
Zn(OH) 2 is amphoteric hydroxide which is soluble in acid to form zinc salts and soluble in strong bases to form complex ions, or zincate:
Zn ( OH ) 2+2H+= Zn2++2H2O
Zn ( OH ) 2+2OH-=[ Zn ( OH ) 4]2-
Zn(OH) 2 is also soluble in NH 3 water to form tetraammine zincate complex ions, while Al(OH) 3 is insoluble in NH 3 water:
Zn ( OH ) 2 +4NH 3 == [ Zn ( NH 3 ) 4 ] 2+ +2OH -
This is also one of the methods for distinguishing Al(OH) 3 and Zn(OH) 2 .
Zn(OH) 2 is easily dehydrated when heated to form white zinc oxide ZnO:
Zn ( OH ) 2 heating Â ZnO+H 2 O
[Zn(OH) 4 ] 2- and [Zn(NH 3 ) 4 ] 2+ dissolve ions and form Zn(OH) 2 under heating or acid addition:
[Zn ( NH 3 ) 4 ] 2+ +2OH - Â heating Â Zn ( OH ) 2 â†“ +4NH 3 â†‘
Both Zn(OH) 2 and ZnO are covalent compounds. Zn(OH) 2 is often used as a filler for papermaking.
ZnO is a famous white pigment commonly known as zinc white. It has the advantage that the H 2 S gas is not blackened because ZnS is also white. When heated, ZnO gradually changes from white to light yellow to lemon yellow. When cooled, the yellow color retreats. Using this property, it is incorporated into paint or added to a thermometer to form a color changing paint or a color changing thermometer.
Due to the astringent nature of ZnO and certain bactericidal ability, it is often used in medicine as an ointment, and ZnO can also be used as a catalyst.
Adding a Na 2 CO 3 solution to the Zn 2+ salt gives a white precipitate of basic zinc carbonate instead of Zn(OH) 2 :
2Zn 2+ +3CO 3 2- +2H 2 O === Zn 2 ( OH ) 2 CO 3 â†“ +2HCO 3 -
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