Animated Solution for Chemistry - Environmental Chemistry: The recommended concentration of fluoride ion in drinking water is up to $1\text{ ppm}$ as fluoride ion is required to make teeth enamel harder by converting $[3{\text{Ca}}_3({\text{PO}}_4{)}_2\cdot \text{Ca}(\text{OH}{)}_2]$ to :

Have you ever looked at a tube of toothpaste or a municipal water report and wondered why fluoride is such a big deal? It turns out that in trace amounts—specifically up to $1\text{ ppm}$—the fluoride ion (${\text{F}}^{-}$) acts as a microscopic superhero for your dental health.

Our teeth are constantly under attack from acids produced by bacteria in our mouths. To defend against this, we need a strong shield. Fluoride provides exactly that by fundamentally altering the chemical structure of our tooth enamel, making it significantly harder and more resistant to decay.

To understand how fluoride works, we first need to look at what our teeth are made of. The hard outer layer of your teeth, the enamel, is primarily composed of a crystalline mineral known as Hydroxyapatite.

The chemical formula for hydroxyapatite is $[3{\text{Ca}}_3({\text{PO}}_4{)}_2\cdot \text{Ca}(\text{OH}{)}_2]$.

If you look closely at the formula, you will notice the hydroxide (${\text{OH}}^{-}$) group at the very end. While hydroxyapatite is quite strong, this hydroxide group is its Achilles' heel. It is relatively susceptible to being dissolved by the acidic environment created when we eat sugary foods.

A Microscopic Battle

When you drink water containing fluoride, a fascinating chemical reaction takes place right on the surface of your teeth. The fluoride ions (${\text{F}}^{-}$) in the water come into contact with the hydroxyapatite crystals.

Because the fluoride ion is similar in size to the hydroxide ion but more electronegative, it can seamlessly slip into the crystal lattice. It effectively kicks out the hydroxide ion and takes its place. This process is a classic example of an ion-exchange or substitution reaction:

The result of this microscopic swap is a brand new mineral called Fluorapatite, which has the formula $[3{\text{Ca}}_3({\text{PO}}_4{)}_2\cdot {\text{CaF}}_2]$.

Fluorapatite is significantly harder and much less soluble in acid than hydroxyapatite. By converting the vulnerable hydroxyapatite into the robust fluorapatite, fluoride effectively armors your teeth against cavities.

Looking back at our options, we can clearly see that the correct formula for the converted enamel is $[3{\text{Ca}}_3({\text{PO}}_4{)}_2\cdot {\text{CaF}}_2]$, which perfectly matches option (c). It is a beautiful example of how a tiny chemical change can have a massive impact on our everyday health!