Which effect do alkalis typically have on human hair?

• A. They have no effect on the structure or texture of hair.
• B. They strengthen hair by decreasing its pH and sealing the cuticle.
• C. They make hair more brittle by increasing its pH and opening the cuticle.
• D. They cause hair to become oily by increasing sebum production.

Answer: (C)

Verified step by step guidance

1. Understand that human hair is primarily composed of a protein called keratin, which has a specific structure maintained by various chemical bonds, including disulfide bonds and hydrogen bonds.
2. Recognize that the pH level affects the hair's structure: acidic conditions (low pH) tend to keep the hair cuticle closed and smooth, while alkaline conditions (high pH) cause the cuticle to open.
3. Know that alkalis are substances with a high pH, meaning they increase the pH when applied to hair, making the environment more basic.
4. When alkalis increase the pH of hair, the cuticle layer opens up, exposing the inner cortex and weakening the hair's structural integrity, which can lead to brittleness and damage.
5. Therefore, alkalis typically make hair more brittle by increasing its pH and opening the cuticle, rather than strengthening it or having no effect.