Acetals can be selectively hydrolyzed back to aldehydes or ketones using aqueous acid. The acid acts as a catalyst, protonating the acetal oxygen, which increases the electrophilic character of the carbonyl carbon, making it more susceptible to nucleophilic attack by water. This results in the formation of a hemiacetal intermediate, which can then be further hydrolyzed to yield the free aldehyde or ketone along with two equivalents of alcohol.