What is the primary ingredient in bleach used for hair?

The primary ingredient in hair bleach is hydrogen peroxide. This chemical agent acts as a powerful oxidizer that breaks down the pigment in hair, specifically melanin, allowing for the lightening effect that bleaching provides. When hydrogen peroxide is applied to the hair, it penetrates the cuticle and initiates the oxidation process, resulting in the lightening of the hair color.

While ammonia is often used in conjunction with hydrogen peroxide to swell the hair cuticle and allow better penetration of the bleach, it is not the primary ingredient responsible for the bleaching action. Calcium carbonate and propylene glycol do not play significant roles in the hair lightening process; calcium carbonate is more commonly used as a bulking agent in products, and propylene glycol is typically employed for its moisturizing properties. Therefore, hydrogen peroxide is the correct choice as it is essential for the actual chemical reaction that lightens hair.