Teeth enamel is largely hydroxyapatite \(\left[\mathrm{Ca}_{5}\left(\mathrm{PO}_{4}\right)_{3} \mathrm{OH}\right]\). When it dissolves in water (a process called demineralization), it dissociates as follows: $$\mathrm{Ca}_{5}\left(\mathrm{PO}_{4}\right)_{3} \mathrm{OH} \longrightarrow 5 \mathrm{Ca}^{2+}+3 \mathrm{PO}_{4}^{3-}+\mathrm{OH}^{-}$$ The reverse process, called remineralization, is the body's natural defense against tooth decay. Acids produced from food remove the \(\mathrm{OH}^{-}\) ions and thereby weaken the enamel layer. Most toothpastes contain a flouride compound such as \(\mathrm{NaF}\) or \(\mathrm{SnF}_{2}\). What is the function of these compounds in preventing tooth decay?

The demineralization process occurs when the hydroxyapatite in tooth enamel dissolves in water or in response to microbial acids. This chemical structure, which makes up the majority of tooth enamel, contains calcium, phosphate, and hydroxyl ions.

During demineralization, these ions are released into the mouth as a result of acidic conditions, commonly created after consuming sugary or starchy foods. Acids from bacterial metabolism can reduce the pH in the oral cavity, triggering the breakdown of hydroxyapatite as the equation shows: \[\mathrm{Ca}_{5}(\mathrm{PO}_{4})_{3} \mathrm{OH} \longrightarrow 5 \mathrm{Ca}^{2+} + 3 \mathrm{PO}_{4}^{3-} + \mathrm{OH}^{-}\].

This results in a weaker and more porous enamel structure, which is susceptible to cavities and other dental issues. Preventive measures like regular brushing and avoiding excessive sugar intake can help combat this process.

Contrary to demineralization, remineralization is a natural repair mechanism of the body to strengthen and restore the mineral content of tooth enamel. Saliva plays a crucial role in this process as it contains calcium and phosphate ions. When pH levels in the mouth rise to a more neutral state, these ions from saliva can reattach to the tooth enamel, in the presence of available hydroxyl groups, to form hydroxyapatite once again.

Maintaining good oral hygiene with adequate fluoride exposure and a balanced diet supports the remineralization process. Foods rich in calcium and phosphate, such as dairy products and leafy greens, can enhance remineralization, helping to reverse early stages of tooth decay and fortify the enamel.

Fluoride is a naturally occurring mineral known for its benefits in oral health. Toothpastes commonly contain fluoride compounds like sodium fluoride (\(\mathrm{NaF}\)) or stannous fluoride (\(\mathrm{SnF}_{2}\)), which are key in preventing tooth decay.

When fluoride is present, it can integrate into the demineralized areas of enamel to form a new, more durable mineral called fluorapatite as shown: \[\mathrm{Ca}_{5}(\mathrm{PO}_{4})_{3} \mathrm{OH} + \mathrm{F}^{-} \longrightarrow \mathrm{Ca}_{5}(\mathrm{PO}_{4})_{3} \mathrm{F} + \mathrm{OH}^{-}\]. This new mineral structure is much less soluble in an acidic environment and therefore more resistant to further demineralization.

Regular use of fluoride toothpaste reinforces tooth enamel and reduces the risk of cavities, making it an essential component in daily dental care routines.