Final answer:
When the pH of a shampoo is excessively alkaline or acidic, it may cause the hair fiber to swell or contract, leading to damage and making it difficult to manage.
Step-by-step explanation:
If the pH of a shampoo is too alkaline (or too acidic), the hair fiber can swell or contract. This is because hair fibers, like many important biological molecules such as proteins and DNA, can be damaged under highly acidic or alkaline conditions. In the case of hair, an overly alkaline shampoo can cause the hair to swell, as it disrupts the chemical structure of the keratin proteins in the hair. This can lead to hair that feels rough, is difficult to manage, and has increased potential for damage and breakage. On the other hand, if a shampoo is too acidic, it could potentially contract the hair fibers, also leading to damage.