Explain the Bronsted-Lowry theory for acids and bases.

(N/A) The Danish chemist,Johannes Bronsted and the English chemist,Thomas $M$. Lowry provided a more general definition of acids and bases.
Definition: According to the Bronsted-Lowry theory,an acid is a substance capable of donating a hydrogen ion $(H^{+})$ and a base is a substance capable of accepting a hydrogen ion $(H^{+})$. In short,acids are proton donors and bases are proton acceptors.
Example $1$: Consider the dissolution of $NH_{3}$ in $H_{2}O$ represented by the following equation:
$NH_{3(aq)} + H_{2}O_{(l)} \rightleftharpoons NH_{4(aq)}^{+} + OH_{(aq)}^{-}$
In this reaction,the water molecule acts as a proton donor (acid) and the ammonia molecule acts as a proton acceptor (base). In the reverse reaction,$H^{+}$ is transferred from $NH_{4}^{+}$ to $OH^{-}$. Here,$NH_{4}^{+}$ acts as a Bronsted acid and $OH^{-}$ acts as a Bronsted base.
Conjugate Acid-Base Pair: An acid-base pair that differs only by one proton is called a 'conjugate acid-base pair'.
$(i)$ $NH_{4}^{+}$ and $NH_{3}$
$(ii)$ $H_{2}O$ and $OH^{-}$
Here,$OH^{-}$ is the conjugate base of the acid $H_{2}O$,and $H_{2}O$ is the conjugate acid of $OH^{-}$. $NH_{4}^{+}$ is the conjugate acid of the base $NH_{3}$,and $NH_{3}$ is the conjugate base of $NH_{4}^{+}$.
Note: $A$ conjugate acid has one extra proton,and each conjugate base has one less proton. If a Bronsted acid is strong,its conjugate base is weak,and vice-versa.
Example $2$: Consider the ionization of hydrochloric acid in water: $HCl_{(aq)} + H_{2}O_{(l)} \rightarrow H_{3}O_{(aq)}^{+} + Cl_{(aq)}^{-}$. Here,$HCl$ acts as an acid by donating a proton to the $H_{2}O$ molecule,which acts as a base.