Chapter 16, Problem 175AP

Tooth enamel is largely hydroxyapatite $\left[{\text{Ca}}_{\text{3}}{\left({\text{PO}}_{\text{4}}\right)}_3\text{OH}\right]$. When it dissolves in water (a process called demineralization), it dissociates as follows:

${\text{Ca}}_{\text{5}}{\left({\text{PO}}_{\text{4}}\right)}_3\text{OH}\to {\text{5Ca}}^{\text{2+}}{\text{+3PO}}_4^{3-}+{\text{OH}}^{\text{-}}$

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 ${\text{NaF or SnF}}_{\text{2}}\text{.}$. What is the function of these compounds in preventing tooth decay?