Tooth enamel is largely hydroxyapatite [Ca3(PO4)3OH]. When it dissolves in water (a proces...

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

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?