the nucleophilic addition of carbonyl is catalysed by an acid.why?

Because addition of acid increases the positive charge density on carbonyl carbon, due to reaction becomes fast. Even weak nucleophile will be able to the addition reaction then on that carbonyl .

Approved

Nucleophillic addition reactions can be of 2 types: Acid catalysed and base catalysed. The acid is often the proton. In these reactions the conjugate acid of the carbonyl group is a better electrophile than the neutral carbonyl group itself. Thus making the attack of any type of nucleoplhile easy.

Approved