CBSE Class 10 Answered

Arrhenius theory:

The Arrhenius acid-base concept classifies a substance as an acid if it produces hydrogen ions H(+) or hydronium ions in water. A substance is classified as a base if it produces hydroxide ions OH(-) in water. This way of defining acids and bases works well for aqueous solutions, but acid and base properties are observed in other settings also.Hence,we needed other theories.

Lewis theory:

 According to this concept, an acid is a species (charged or uncharged), which can accept a pair of electrons and a base is a species (charged and uncharged), which can donate a pair of electrons towards the formation of a covalent bond. Thus, a Lewis acid is an electrophile, and a Lewis base is a nucleophile. For example, in the reaction