Question

When Al is added in KOH solution:
A. No action takes place
B. Oxygen is evolved
C. Water is produced
D. Hydrogen is evolved

Answer 1

Hint: The reaction of aluminium in KOH is an example of a redox reaction. The Aluminium metal is oxidized to aluminium ion with an oxidation number of +3 and the hydrogen in KOH is reduced from an oxidation number of +1 to zero by releasing hydrogen gas.

Complete step-by-step answer:
When metallic elements such as sodium or potassium from groups IA and IIA of the periodic table come in direct contact with water, the corresponding metal hydroxide and hydrogen are formed. Such a reaction only proceeds with aluminium in the presence of a strong alkaline compound, such as sodium hydroxide or potassium hydroxide because these metals have a very thin passive layer of \[A{l_2}{O_3}\] on their surface such that they prevent the direct attack of water molecules.
\[2Al + 6KOH \to 3{H_2} + 2{K_3}Al{O_3}\]
This is an oxidation-reduction reaction or redox reaction because aluminium metal gets oxidized to its respective ion by losing electrons while the hydrogen in potassium hydroxide of oxidation number +1 gain an electron and gets reduced to zero oxidation number, thereby releasing hydrogen gas.
Reaction of aluminium with potassium hydroxide is a very common process as it is also used to prepare alum from aluminium metal.
\[2Al + 2KOH + 6{H_2}O \to 3{H_2} + 2KAl{(OH)_4}\]
From these reactions, we can observe the evolution of hydrogen gas when aluminium reacts with potassium hydroxide.

Hence, the correct option is (D).

Note: In this reaction, there is almost no consumption of potassium hydroxide because the potassium aluminate formed as a side product in the hydrogen generation undergoes a decomposition reaction that results in regeneration of the alkali.