WE ALL KNOW THAT EATING candy and other foods loaded with sugar causes tooth decay, but have you ever wondered
why? We also know that brushing and flossing
can prevent tooth decay, but how does this
actually work? From the process that keeps
your teeth hard as a rock to the decay that
occurs in many people and the materials that
dentists use to repair the damage, there are
different types of chemicals and chemical reactions going on inside your mouth.
What’s in a tooth?
Like bones, the hard parts of a tooth consist
of a combination of mineral, proteins, and
some water. Here, “mineral” means a compound that is solid and inorganic—not made
of carbon-based molecules—with a definite
chemical composition and an ordered atomic
The white, outer layer of the tooth, or
enamel, is 96% mineral (Fig. 1). This high
fraction of mineral is what makes enamel
the hardest substance in the body; it is basically a kind of rock. Known as hydroxyapatite
3(OH)), the mineral consists of calcium (Ca2+) and phosphate (PO43–) ions.
Hydroxyapatite also is present in dentin, the
part of the tooth under the enamel and in the
root of the tooth (Fig. 1), but hydroxyapatite
makes up only about 70% of those tissues,
the remainder being 20% organic material
and 10% water. Because it is less mineralized,
dentin is softer than enamel.
Despite their rock-hard texture, your teeth
are alive, and the hydroxyapatite is dynamic.
Small amounts of calcium and phosphate
ions dissolve from enamel hydroxyapatite into
the saliva—a process called demineralization
(Fig. 2)—when the saliva’s pH drops below 5. 5.
Normally, this happens when
you eat acidic foods, such as
pineapple and strawberries,
which have a pH below 4.
Fortunately, the effect of
those acidic foods is tempo-
rary. Saliva has a pH ranging
from 6.2 to about 7. 4, and
it contains chemical compounds, called buf-
fers, that resist pH change. So, the pH on the
surface of the tooth quickly returns to normal,
above 5. 5, and once that happens, the calcium
and phosphate ions that are present in saliva
are remineralized back to hydroxyapatite.
When you don’t eat food, mineralization
and demineralization still occur, but at low
rates—only relatively few ions
form hydroxyapatite or sepa-
rate from your teeth—and the
rates of both mineralization
and demineralization are equal
(Fig. 3). This state of balance
is an example of a chemical
equilibrium, and it can be
expressed as follows:
3(OH) ⇌ 5 Ca2+ + 3 PO43–+ OH–
But what if your teeth are not healthy?
Tooth decay develops when hydroxyapatite’s mineralization cannot keep up with
demineralization. This is called an unstable
equilibrium, because the number of calcium
and phosphate ions that are released in the
mouth from the dissolution of hydroxyapatite is higher than the number of calcium and
phosphate ions that react with each other to
form hydroxyapatite. Such an unstable equilibrium happens when teeth are exposed to
low pH for extended periods of time.
Figure 1. The structure of a tooth
If our saliva’s pH drops
below 5. 5, our teeth start
losing minerals, and
tooth decay may ensue.
Brushing teeth helps
maintain a pH of 5. 5 by
removing acidic food
leftovers on our teeth.
8 ChemMatters | OCTOBER/NOVEMBER 2015 www.acs.org/chemmatters
By David Warmflash