Group 2: Question 5

Worked solution

Recalling the solubility trend of Group 2 hydroxides

$\text{Mg(OH)}_2$ is only very sparingly soluble in water (it is the white suspension in “milk of magnesia”). Moving down the group, the hydroxides become progressively more soluble: $\text{Ca(OH)}_2$ (limewater) is slightly soluble, $\text{Sr(OH)}_2$ is more soluble still, and $\text{Ba(OH)}_2$ is soluble enough to make a fairly concentrated solution (“baryta water”).

Linking solubility to alkalinity

All the Group 2 hydroxides are strong bases. Whatever amount does dissolve is fully (100%) dissociated into $\text{M}^{2+}$ and $\text{OH}^-$ ions. Since $\text{Ba(OH)}_2$ dissolves to a much greater extent than $\text{Mg(OH)}_2$, a saturated solution of $\text{Ba(OH)}_2$ contains a much higher concentration of $\text{OH}^-$ ions, and so is more strongly alkaline (higher pH) than a saturated solution of $\text{Mg(OH)}_2$.

So: solubility increases down the group and alkalinity of the saturated solution increases down the group, for the same underlying reason. More dissolved hydroxide means more $\text{OH}^-$ ions in solution.

Why the other options are wrong

• A reverses the solubility trend. (This description actually matches the Group 2 sulfates, whose solubility decreases down the group. A common point of confusion.)
• C is wrong because the hydroxides are not “essentially insoluble” as a group. Even $\text{Mg(OH)}_2$ dissolves slightly, and solubility increases substantially by the time you reach $\text{Ba(OH)}_2$.
• D correctly states that solubility increases down the group, but wrongly calls $\text{Ba(OH)}_2$ a “weak base”. Being more or less soluble does not make a hydroxide weak or strong; a strong base is one that fully dissociates into ions, which all the Group 2 hydroxides do.

Final answer

B. Solubility (and hence alkalinity of the saturated solution) increases down the group, from $\text{Mg(OH)}_2$ to $\text{Ba(OH)}_2$.