Chapter 16, Problem 16.173QP

Tooth enamel is largely hydroxyapatite [Ca₃(PO₄)₃OH]. When it dissolves in water (a process called demineralization), it dissociates as follows:

${\text{Ca}}_{\text{5}}{({\text{PO}}_{\text{4}})}_{3}OH\to {\text{5Ca}}^{\text{2+}}{\text{+3PO}}_4^{3-}+{\text{OH}}^{-}$

The reverse process, called remineralization, is the body’s natural defense against tooth decay. Acids produced from food remove the OH⁻ ions and thereby weaken the enamel layer. Most toothpastes contain a fluoride compound such as NaF or SnF₂. What is the function of these compounds in preventing tooth decay?