Which chemical is commonly used for perming hair?

Ammonium thioglycolate is the chemical most commonly used for perming hair because it serves as a reducing agent that breaks down the disulfide bonds in the hair. This process allows the hair to be reshaped into curls or waves when wrapped around perm rods. Once the hair is treated with ammonium thioglycolate, it can be set into a new shape, which is then fixed in place using an oxidizing agent, typically hydrogen peroxide, to reform the disulfide bonds in their new configuration.

This chemical formulation is specifically designed for the perming process, making it effective, safe, and reliable for changing the hair's structure. Other chemicals listed, such as hydrogen peroxide, while vital in the overall perming process, play a different role primarily in neutralizing and fixing the curl. Calcium hydroxide is associated with hair relaxers and different chemical treatments, while etidronic acid does not have a relevant application in hair perming. Thus, ammonium thioglycolate stands out as the correct choice in this context.