The critical pH for hydroxyapatite dissolution is:

• A. 4.0
• B. 5.5
• C. 6.8
• D. 7.0

Answer: (B)

The idea being tested is the tipping point at which enamel mineral (hydroxyapatite) begins to dissolve in the mouth. This critical pH is about 5.5. When the surrounding fluid has a pH above roughly 5.5, saliva is typically supersaturated with calcium and phosphate, which supports remineralization and keeps enamel stable. If the pH drops below about 5.5, the environment becomes undersaturated with respect to hydroxyapatite, so dissolution dominates and demineralization of the enamel occurs. The values around neutral to slightly acidic (6.8–7.0) do not favor dissolution, while a much more acidic environment (like 4.0) would promote dissolution, but the standard threshold used to describe this balance is around 5.5.