Question

Which is base and not alkali?
(a)- $NaOH$
(b)- $KOH$
(c)- $Fe{{(OH)}_{3}}$
(d)- $N{{H}_{4}}OH$

Answer 1

Hint: A base is that substance that has a tendency to donate the hydroxyl ion when dissolved in water. The base is differentiated from alkali as alkalis are soluble in water. So, the bases that are not soluble in water are not alkalis.

Complete step by step answer:
A base is that substance that has a hydroxyl group ($O{{H}^{-}}$) and when the substance is dissolved in water it dissociates into a cation and hydroxyl ion.
So, the alkalis are those bases that can be dissolved in water. If the base is insoluble in water then it is not an alkali.
So, $NaOH$ is a strong base, and when it is dissolved in water, it dissociates into sodium cation and hydroxyl anion. Being a strong base it dissociates completely. The reaction is given below:
$NaOH\to N{{a}^{+}}+O{{H}^{-}}$
So, it is an alkali.

$KOH$ is also a strong base. When it is dissolved in water, it dissociates into potassium cation and hydroxyl anion. Being a strong base it dissociates completely. The reaction is given below:
$KOH\to {{K}^{+}}+O{{H}^{-}}$
So, it is an alkali.

$Fe{{(OH)}_{3}}$ is a base, but it is not soluble in water. So, it is not an alkali.
$N{{H}_{4}}OH$ is a weak base and when dissolved in water, it dissociates to a small extent. It forms ammonium cation and hydroxyl anion. The reaction is given below:
 $N{{H}_{4}}OH\rightleftharpoons NH_{4}^{+}+O{{H}^{-}}$
So, the correct answer is “Option C”.

Note: It should be noted that all bases are not alkali, but all alkalis are bases. Some other examples that are bases but not alkalis are zinc hydroxide, copper oxide, etc.