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

Hint: 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.