Rank the following hydroxides in order of increasing aqueous

basicity:$\mathit{A}\mathit{l}{\left(OH\right)}_{\mathbf{3}}\mathbf{,}\mathit{B}{\left(OH\right)}_{\mathbf{3}}\mathbf{,}\mathit{I}\mathit{n}{\left(OH\right)}_{\mathbf{3}}$

Aqueous basicity is determined by the size of the metals. The size of metals increases when we move down the group. Since each compound has a hydroxyl group, the properties of each metal are different. Boron lies above aluminum and indium. The order of these metals in the group is boron, aluminum and then indium. Indium lies below aluminum and so has larger size than both boron and aluminum.

Aqueous basicity is related to the atomic size. When atomic size is increased, the bond length of the metal with oxygen is also increased. Thus it is very easy to lose the hydroxyl group. This means that the basicity is also increased.

So the increasing order of aqueous basicity is as follows:

$B(OH{)}_3<Al(OH{)}_3<In(OH{)}_3$

Thus the order is given above.