Chemistry Reference
In-Depth Information
Amphoteric hydroxides and oxides
The hydroxides of metals are basic and they react
with acids to form salts (Chapter 8, p. 124). The
hydroxides of some metals, however, will also react
with strong bases, such as sodium hydroxide, to
form soluble salts. Hydroxides of this type are said
to be
amphoteric
. For example,
zinc
hydroxide
Zn(OH)
2
(
aq
)
+
+
hydrochloric
acid
2HCl(
aq
)
zinc
chloride
ZnCl
2
(
aq
)
+
+
water
2H
2
O(
l
)
→
●
●
→
and
zinc
hydroxide
Zn(OH)
2
(
aq
)
+
+
sodium
hydroxide
2NaOH(
aq
)
sodium zincate
Na
2
Zn(OH)
4
(
aq
)
→
●
●
→
a
Iron(
iii
) hydroxide is precipitated.
Figure 10.9
b
Copper(
ii
) hydroxide is precipitated.
Other amphoteric hydroxides are lead hydroxide
(Pb(OH)
2
) and aluminium hydroxide (Al(OH)
3
).
We can use this sort of behaviour to help identify
metal cations, as their hydroxides are soluble in
strong bases.
Both aluminium and zinc metals will also react
readily with moderately concentrated acids and alkalis.
zinc
Zn(
s
)
The ionic equation for this reaction is:
iron(
iii
) ions
Fe
3+
(
aq
)
+
+
hydroxide ions
3OH
−
(
aq
)
iron(
iii
) hydroxide
Fe(OH)
3
(
s
)
→
●
→
Table 10.5 shows the effects of adding a few drops
of sodium hydroxide solution to solutions containing
various metal ions, and of adding an excess. The
colours of the insoluble metal hydroxides can be used
to identify the metal cations present in solution. In
some cases the precipitate dissolves in excess hydroxide,
owing to the amphoteric nature of the metal
hydroxide. This amphoteric nature can also be used to
help identify metals such as aluminium and zinc.
Table 10.5
The effect of adding sodium hydroxide solution to solutions
containing various metal ions.
+
hydrochloric
acid
2HCl(
aq
)
zinc chloride
ZnCl
2
(
aq
)
+
hydrogen
H
2
(
g
)
→
●
●
→
+
+
zinc
Zn(
s
)
+
+
sodium
hydroxide
2NaOH(
aq
)
+
+
water
2H
2
O(
l
)
sodium
zincate
Na
2
Zn(OH)
4
(
aq
)
+
+
hydrogen
H
2
(
g
)
→
●
●
→
It should be noted that the oxides of the metals
used as examples above are also amphoteric.
Aluminium oxide and zinc oxide will react with
both acids and alkalis.
zinc
oxide
ZnO(
s
)
Metal ion present
in solution
Effect of adding sodium hydroxide solution
A few drops
+
+
hydrochloric
acid
2HCl(
aq
)
zinc
chloride
ZnCl
2
(
aq
)
+
+
water
H
2
O(
l
)
→
●
●
→
An excess
Aluminium
White precipitate of
aluminium hydroxide
Precipitate dissolves
Calcium
White precipitate of
calcium hydroxide
Precipitate does not
dissolve
zinc
oxide
ZnO(
s
)
+
sodium
hydroxide
2NaOH(
aq
)
+
water
H
2
O(
l
)
sodium zincate
Na
2
Zn(OH)
4
(
aq
)
→
●
●
→
Copper(
ii
)
Blue precipitate of
copper(
ii
) hydroxide
Precipitate does not
dissolve
+
+
Iron(
ii
)
Green precipitate of
iron(
ii
) hydroxide
Precipitate does not
dissolve
Certain non-metal oxides are classified as neutral.
These oxides do not react with acids or bases.
Examples include carbon monoxide (CO),
nitrogen(i)
ii
) oxide (nitrogen monoxide, NO), and
nitrogen(i)
i
) oxide (nitrous oxide or dinitrogen
oxide, N
2
O).
Iron(
iii
)
Brown precipitate of
iron(
iii
) hydroxide
Precipitate does not
dissolve
Zinc
White precipitate of
zinc hydroxide
Precipitate dissolves
Chromium(
iii
)
Light green
precipitate
Precipitate dissolves
