GROUPS I AND II 131
+ CO 2 -*HCOJ
The reaction between Ca(OH) 2 + CO 2 to produce sparingly
soluble CaCO 3 is the common test for carbon dioxide.
Beryllium hydroxide is obtained as a white gelatinous precipitate
when OH~ ions are added to a solution of a beryllium salt. It is
only sparingly soluble in water, and is weakly basic, dissolving in
strong acids to give the hydrated beryllium ion [Be(H 2 O) 4 ]^2 + , but
also dissolving in solutions containing the hydroxide ion to give the
tetrahydroxoberyllateill) ion [Be(OH) 4 ]^2 " ; addition of acid first re-
precipitates the hydroxide Be(OH) 2 (as a white gelatinous hydrated
precipitate) and then re-dissolves it to give the hydrated ion ; hence
we have the sequence*
[Be(H 2 0) 4 ]2+ ±Be(OH)H 2 [Be(OH) 4 ]^2 -
L(H 2 o) 2 |
H
This behaviour distinguishes beryllium hydroxide from the other
hydroxides of Group II which are not amphoteric; this amphoterism
is also shown by aluminium hydroxide in Group III, and it has been
discussed more fully in Chapter 2, where we saw it as characteristic
of small ions of high charge, i.e. Be2+ and A1^3 +.
The other Group II hydroxides are sparingly soluble in water,
the solubility increasing down the group ; magnesium hydroxide is
precipitated only by an appreciable concentration of hydroxide ion
(not by ammonium hydroxide in presence of ammonium chloride)
and the others are not precipitated.
SUMMARY OF PROPERTIES OF HYDROXIDES
Element Li Na K Rb Cs
MOH ^ soluble
Base strength increasing
Element
M(OH) 2
Be Mg Ca
Insoluble
Amphotci ic
Sr
Solubility
Base strength
Ba
increasing
increasing
- The species involved are more complicated than this sequence indicates, see
note on p. 46; the simplified representation is. however, quite adequate.
