Question

Which of the following hydroxides is insoluble in water?
(A) $Be{\left(OH\right)}_2$
(B) $Mg{\left(OH\right)}_2$
(C) $Ca{\left(OH\right)}_2$
(D) $Ba{\left(OH\right)}_2$

Answer 1

Hint: Take a look at the given metal hydroxides and locate the position of metals in the periodic table. We know as we move from top to bottom in a group, atomic size increases and as we go from left to right in a periodic table, atomic size decreases. Think about some similar periodic trends and associate the solubility concept to get the answer.

Complete step by step solution:
-Let’s start by identifying metals given in the hydroxides.
(A) $Be{\left(OH\right)}_2$ has beryllium metal.
(B) $Mg{\left(OH\right)}_2$ has magnesium metal.
(C) $Ca{\left(OH\right)}_2$ has calcium metal.
(D) $Ba{\left(OH\right)}_2$ has barium metal.
-We can see that all the four metals belong to the same group in the periodic table. They all are alkaline earth metals.
-Alkaline earth metals are arranged in the order: beryllium, magnesium, calcium, strontium and barium according to their atomic number in the periodic table.
-We are aware of the trends in the periodic table. As we move from top to bottom in a group, atomic size and ionic size increases and electropositivity increases as well due to higher shielding effect as we move down the group.
-Electropositivity is the tendency of an atom to donate its electrons in order to attain stability.
-Higher the electropositivity, higher will be the ionic nature of the compound formed and higher will be the solubility.
-As we move down the group, atomic radius increases so bond length in compounds also decreases due to large atomic size and bond strength will decrease. So the compounds can easily get dissolved in water.
-Therefore, $Ba{\left(OH\right)}_2$ is the most water-soluble hydroxide in the group and $Be{\left(OH\right)}_2$ is the most water-insoluble hydroxide in the group.

Therefore, the answer is option (A).

Note: Remember as the atomic size increases, bond length increases and bond strength decreases which in turn makes the compound easily soluble in water. Higher the electropositivity and higher the electronegativity, more is the ionic character of the compound and greater will be its solubility in water.