HCl (hydrochloric acid) is a strong acid. Arrhenius theory, Brønsted-Lowry acid base theory, and Lewis acid-base theory are the theories which explain the acid nature of hydrochloric acid. According to Arrhenius theory , a compound which when dissolved in water dissociates and furnishes H+ ions is an Arrhenius acid. According to Bronsted-Lowry theory of conjugated acid-base pairs, the H3O+ forms a conjugate base which is water (weakly basic). Here HCl donates H+ to water hence it can be considered as the Bronsted acid. According to Lewis Theory , any compound which can accept lone pair of electrons from a donor atom/molecule/group is an acid. H+ being a cation easily accepts lone pair of electrons. Thus we can say that HCl is a strong acid as it readily and easily dissociates in water and furnishes H+ ions.