North Carolina Cosmetic Arts Practice Exam

Ammonium thioglycolate is the chemical commonly used in hair relaxers due to its ability to break the disulfide bonds in the hair's keratin structure. This process allows the hair to be reshaped and straightened. It works effectively for achieving a smoother and straighter hairstyle by altering the natural curl pattern of the hair.

The other options do have their own specific uses in the beauty and cosmetic industry. Hydrogen peroxide, for example, is primarily used as a bleaching agent or in hair color formulations but is not suitable for hair relaxing. Ethyl alcohol is commonly used as a solvent or astringent in various cosmetic products, while acetic acid is known for its role in hair conditioning and maintaining pH balance but does not serve the purpose of chemically relaxing hair. Thus, the key role that ammonium thioglycolate plays in effectively and safely relaxing hair makes it the correct choice in this context.