Illinois Cosmetology Practice Exam 2025 – 400 Free Practice Questions to Pass the Exam

Sodium hydroxide is primarily used in hair straightening treatments because it acts as a strong alkali that alters the structure of the hair. This chemical is part of a group known as relaxers, which break down the disulfide bonds in the hair’s keratin structure. By breaking these bonds, sodium hydroxide allows the hair to be reshaped into a straighter form when combined with heat during the treatment process.

This is particularly effective for coarse or tightly curled hair types, as it can provide a more manageable and straightened appearance. The strength of sodium hydroxide necessitates its careful application, as it can cause damage if not used properly, which is a critical aspect for cosmetology students to understand.

Other chemicals listed, such as glycolic acid, ammonium thioglycolate, and hydrogen peroxide, are used in different hair treatments and processes, but sodium hydroxide's function in permanently straightening hair sets it apart as the primary choice for chemical hair straightening. Glycolic acid is primarily an exfoliant, ammonium thioglycolate is often used for permanent waving, and hydrogen peroxide is typically associated with bleaching and hair lightening.