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 bonds in hair that are most readily affected by moisture and heat. They form between polar parts of the keratin proteins in the cortex and help hold the hair’s shape. Water molecules compete with these bonds for hydrogen-sharing interactions, effectively breaking them when hair is wet. Thermal heat increases molecular motion, which also weakens these attractions. When the hair dries and cools, the hydrogen bonds reform, returning the hair to its original shape.

Salt bonds are ionic links that can be disrupted by changes in pH (alkaline or acidic conditions) and can be affected by moisture, but they’re not primarily broken by water or ordinary styling heat. Disulfide bonds are strong covalent links between cysteine amino acids and require chemical reducing agents or more intense processing to break. Peptide bonds are the covalent bonds in the protein backbone and aren’t broken by typical water exposure or styling heat.