Which bonds in the hair are weakened or broken down by water or thermal heat?

• A. Hydrogen bonds
• B. Salt bonds
• C. Disulfide bonds
• D. Peptide bonds

Answer: (A)

Hydrogen bonds are the weakest links in hair, and they are the ones most readily disrupted by moisture and heat. When hair gets wet, water molecules form new hydrogen bonds with the keratin, which competes with and breaks the existing hydrogen bonds between the protein chains. Heat increases molecular motion, making it easier for these bonds to break as well. Once the moisture or heat source is removed and the hair cools, these hydrogen bonds can reform, which is why hair can regain its shape unless a chemical process has altered it.

Salt bonds are ionic bonds that respond to changes in pH, not primarily to water or heat alone. Disulfide bonds are strong covalent bonds between cysteine units and are broken mainly by chemical reducers (and can be reshaped by heat only in conjunction with those chemicals). Peptide bonds form the backbone of the protein and require much stronger chemical changes to break.

So, the bonds weakened by water or thermal heat are hydrogen bonds.