What is meant by strong bases and weak bases? Classify the following into strong bases and weak bases:
 $ N{H_4}OH $ , $ Ca{\left( {OH} \right)_2} $ ​, $ NaOH $ , $ KOH $ , $ Mg{\left( {OH} \right)_2} $

Hint: A base refers to a substance which in water solution tends to be slippery to the touch, bitter in taste and changes the colour of indicators (i.e., turns the red litmus paper to blue). Base reacts with the acids in order to form salts.

Complete step by step solution:
A strong base refers to a base that completely dissociates in an aqueous solution. Strong base ionize in water and yield one or more hydroxide ions (i.e. $ O{H^ - } $ ) per molecule of the base. On the other hand, a weak base refers to a base which partially dissociates in an aqueous solution and ionizes in water partially and thus, yields very less amount of hydroxide ions. The basic nature of the species is inversely proportional to the stability of the species ion. Let us discuss each one of the bases given in the question.
1) $ N{H_4}OH $ : It is a weak base since only 0.4% of $ N{H_4}OH $ dissociates in aqueous solution.
2) $ Ca{\left( {OH} \right)_2} $ : It is a weak base since it does not dissociate completely in aqueous solution.
3) $ NaOH $ : It is a strong base since it dissociates completely in aqueous solution.
$ NaOH \to N{a^ + } + O{H^ - } $
4) $ KOH $ : It is a strong base since it dissociates completely in aqueous solution.
$ KOH \to {K^ + } + O{H^ - } $
5) $ Mg{\left( {OH} \right)_2} $ : It is a weak base since it does not dissociate completely in aqueous solution.
Hence, $ N{H_4}OH $ , $ Ca{\left( {OH} \right)_2} $ ​and $ Mg{\left( {OH} \right)_2} $ are weak bases while $ NaOH $ and $ KOH $ are strong bases.

Note:
Base strength of any species is its capability to accept the $ {H^ + } $ from another species. Greater the ability of any species to accept $ {H^ + } $ from another species, the greater is its base strength.