Question

Find an odd one out (valency is not a criterion).
a) $Al{{(OH)}_{3}}$
b) $Pb{{(OH)}_{2}}$
c) $Mg{{(OH)}_{2}}$,
d) $Zn{{(OH)}_{2}}$

Answer 1

Basic hydroxides are those which release hydroxide ion $(O{{H}^{-}})$ in water. Amphoteric hydroxides are those which act as both acid as well as base depending on the $pH$ of the solution. The hydroxides of s-block elements are basic in nature.

Complete answer:
Magnesium is an alkaline earth metal (s-block element) and hence $Mg{{(OH)}_{2}}$ is basic hydroxide. It means that it releases hydroxide ion $(O{{H}^{-}})$ in water or we can also say that it will undergo a neutralization reaction with acid and produce salt and water. The neutralization reaction for $Mg{{(OH)}_{2}}$ is illustrated as follows:
$2HCl+Mg{{(OH)}_{2}}\to MgC{{l}_{2}}+2H2O$
On the other hand, all remaining hydroxides i.e. $Al{{(OH)}_{3}}$ ,$Pb{{(OH)}_{2}}$ and $Zn{{(OH)}_{2}}$ are amphoteric hydroxides. It means that they can both release and accept hydroxide ions $(O{{H}^{-}})$ depending on the $pH$ of the solution. Let us consider the example of $Al{{(OH)}_{3}}$. This hydroxide undergoes a neutralization reaction in low $pH$solution while accepting hydroxide ion $(O{{H}^{-}})$ from high $pH$ solution.
In low $pH$ solution, the following reaction occurs:
$3HCl(aq.)+Al{{(OH)}_{3}}\to AlC{{l}_{3}}+3{H_2}O$
In the above reaction, $Al{{(OH)}_{3}}$ act as a base and provide salt and water (i.e. neutralization).
In high $pH$solution, the following reaction occurs:
$Al{{(OH)}_{3}}+O{{H}^{-}}\to {{\left[ Al{{(OH)}_{4}} \right]}^{-}}$
In the above reaction, $Al{{(OH)}_{3}}$ act as an acid and hence accept hydroxide ion $(O{{H}^{-}})$ from the solution.
Since $Mg{{(OH)}_{2}}$ is a basic hydroxide while all other are amphoteric, hence $Mg{{(OH)}_{2}}$ is odd one out from others.

So, the correct answer is “Option c”.

Note:
It is important to note that $Mg{{(OH)}_{2}}$ is a basic hydroxide in contrast to all others which are amphoteric in nature. Magnesium is alkaline earth metal and hence forms basic hydroxide. Basic hydroxide releases hydroxide ion $(O{{H}^{-}})$ in water while amphoteric hydroxide can both release and accept hydroxide ion $(O{{H}^{-}})$ depending on the $pH$ of the solution.