What kind of bonds are easily broken by water or heat and can reform when hair cools or dries?

The correct answer is hydrogen bonds, which play a crucial role in the structure of hair. These bonds are relatively weak and can be easily disrupted by heat and water. When hair is subjected to moisture, the hydrogen bonds break, allowing for changes in the hair's form or style. For example, when hair is wet, it becomes more malleable, enabling styling techniques such as curling or straightening. Once the hair dries or cools, hydrogen bonds reform, allowing the hair to retain its new shape.

Hair's structure primarily relies on hydrogen bonds, particularly in the keratin proteins that make up the hair shaft. This flexibility is what makes hydrogen bonds unique compared to other types of bonds found in hair, such as disulfide bonds and ionic bonds, which are much stronger and not easily broken by heat or water. Disulfide bonds, for instance, provide significant strength and stability to the hair's structure but require more intense processes like chemical treatments to break. Therefore, the dynamic nature of hydrogen bonds is essential for styling and temporary alterations in hair.