What is the main ingredient in most relaxers used in cosmetology?

The primary ingredient in most relaxers used in cosmetology is sodium hydroxide. This chemical serves as a powerful alkaline agent that alters the structure of the hair. Sodium hydroxide works by breaking the disulfide bonds in the keratin of the hair, allowing it to be straightened. Its strong alkaline properties make it effective for providing the desired results in hair that is naturally curly or wavy.

Other options, while related to hair treatments, do not represent the main ingredient in relaxers. Sodium bicarbonate is commonly used as a mild exfoliant or pH balancer but does not possess the same chemical Reactivity as sodium hydroxide regarding hair structure. Calcium carbonate is primarily used in other cosmetic products and serves a different function in hair care. Ammonium thioglycolate, on the other hand, is utilized in perm solutions for creating curls rather than in relaxers where the goal is to straighten the hair. Thus, sodium hydroxide stands out as the key ingredient for hair relaxers, making it the correct answer.