Final answer:
Acid perms use an acidic solution along with a basic reducing substance to perform the processing action.
Step-by-step explanation:
Acid perms use an acidic solution to perform the processing action. The acidic solution helps to break down the disulfide bonds in the hair, allowing it to be reshaped. This is achieved by adding a basic reducing substance, usually ammonium thioglycolate, to the hair. The reducing substance reduces and ruptures the disulfide cross-links, allowing the hair to be reshaped.