Question

Why is the solubility of alkaline earth metal hydroxides less than alkali metal hydroxdes?

Answer 1

Solubility of the Hydroxides

Group II metal hydroxides become more soluble in water as you go down the column. This trend can be explained by the decrease in the lattice energy of the hydroxide salt and by the increase in the coordination number of the metal ion as you go down the column.

Element               Lattice energy(kJ/mol)            Coordination number

Be                               3620                                          4
Mg                               2998                                          6
Ca                                2637                                         6                
Sr                                 2474                                         8
Mg                                2330                                         8

The larger the lattice energy the more energy it takes to break the lattice apart into metal and hydroxide ions. Since the atomic radii increase down the group it makes sense that the coordination numbers also increases because the larger the metal ion the more room there is for water molecules to coordinate to it. 

The following examples illustrate this trend:

Magnesium hydroxide appears to be insoluble in water. However, if it is shaken in water and filtered, the solution is slightly basic. This indicates that there are more hydroxide ions in solution than there were in the original water. This is because some magnesium hydroxide has dissolved.Calcium hydroxide solution is referred to as "lime water". A liter of pure water will dissolve about 1 gram of calcium hydroxide at room temperature.Barium hydroxide is soluble enough to produce a solution with a concentration around 0.1 mol dm-3 at room temperature