Question

what is meant solubility of hydroxide ?

Answer 1

Basic character of hydroxide: The hydroxides of alkali metals behave as strong base due to their low ionization enthalpies, and so they can loose electrons and OH- ions.

Due to the low I.P the M-O bond in M-O-H can easily break giving M+ and  OH- ions. as; MOH ---> M+  +  OH-  (basic nature).

As we move down the group from Li, Na to Cs, the I.P decrease and so it becomes more easier to loose electrons and hence the basic charetcter increases down the group.

Solubility of Hydroxides: All the hydroxides are highly soluble in water except LiOH, it is because all Li is less reactive towards water as it has lowest -ve value of standard electrode potential. 

While in case of other hydroxides, they easily ionize in water.

Stability of Hydrides: All the alkali metals react with hydrogen to form colourless ionic hydrides, as 2M +H2 --> 2M+H- 

But, the stability of these hydrides decreases as we move down the group because, the lattice energy of these hydrides decreases down the group as the size of metal cation increases and thus stability of hydrides decreases from LiH to CsH.

Answer 2

Magnesium hydroxide, Mg(OH)2 , is only sparingly soluble in aqueous solution, which means that it does not dissociate completely to form magnesium cations, Mg2+ , andhydroxide anions, OH− . Instead, an equilibrium is established between the undissolved solid and the dissociated ions.
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