Diseases and Conditions: Cavities/Tooth Decay.
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Scientific Affairs, American Dental Association,
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statement-on-dental-amalgam [accessed July
Gabert, S. So You Want to Be… A Dentist? Her
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David Warmflash, M.D., is a science writer who
lives in Portland, OR. His latest ChemMatters
article, “Left Life? Right Life? Chirality in Action,”
appeared in the April/May 2015 issue.
resistant to acid than enamel and demineralizes at a pH of 6.2 or lower. Once there is a
hole in the enamel, the dentin and the rest of
the tooth are in a lot of trouble.
Treating tooth decay
When tooth decay has advanced too far to
be reversed, the decayed, infected part of the
tooth needs to be removed, and the remaining
hole needs to be filled. If you have had any
cavities, you may know that dentists usually
fill them with something called composite
resins. This substance is composed of chainlike molecules, called polymers, which are
made of many similar units, called monomers,
which are linked together.
Composites have been around since the
mid-20th century. However, for decades,
dentists preferred to fill teeth with a metal
alloy, called an amalgam, which is made of
mercury, silver, tin, copper, and tiny amounts
of other metals, because the early composites
did not bond well to dentin.
But there are some problems with amalgam. One of them is that sometimes dentists
remove not only the damaged part of the
tooth, but also some healthy material along
with it, because the place for the filling has to
be shaped just right so that the packed amalgam cannot move.
Another problem is that because metal
blocks X-rays, having amalgam fillings in your
mouth can make it difficult for a dentist to
get a useful 3-dimensional panoramic X-ray
picture, in contrast to composite fillings which
the X-rays can penetrate. Also, amalgam
needs to be disposed of properly, because it
contains mercury. So, today, most dentists
use composite resins instead of amalgams.
Precipitation and dissolution of minerals,
buffers, ions, and polymers—these topics are
probably part of your chemistry curriculum in
school. Now, you can tell your friends and
teachers how they are also at work inside
Acids and bases in
Even though the pH of blood throughout
your body is constant, it can change more
significantly in some parts of your body. In
the mouth, saliva has a pH ranging from 6.2
to about 7. 4, and, as mentioned earlier, it contains chemical compounds, called buffers, that
resist pH change. The carbonic acid (H2CO3)/
bicarbonate (HCO3–) buffer is the major buffer
When food enters the mouth, it is accompanied by a subsequent decrease in pH. The drop
in pH is caused by increased concentrations
of hydrogen ions, due to
lactic acid being produced
by bacteria when they ferment carbohydrates. To
counter this drop in pH, the
salivary ducts secrete more
bicarbonate ions, which help
neutralize the increased amount of hydrogen
ions produced by the bacteria. In saliva, as in
the body, the following equilibria exist:
CO2 + H2O ⇄ H2CO3 ⇄ HCO3– + H+
A rise in bicarbonate ion concentration
conveniently removes the increased amounts
of hydrogen ions produced by bacteria, so the
above equilibrium (H2CO3 ⇄ HCO3– + H+)
shifts to the left to produce more carbonic
acid. But the concentration of carbonic
acid in the mouth is maintained at a nearly
constant level, so the excess carbonic acid
needs to be removed. To make this happen,
the other equilibrium (CO2 + H2O ⇄ H2CO3)
shifts to the left, and more carbon dioxide
is produced. Ultimately, this excess carbon
dioxide diffuses out from the saliva.
In this way, excess hydrogen ions are
effectively neutralized and removed and,
over time, new equilibria are established. 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, reducing the risk of erosion of
The problem occurs when the pH at the
tooth surface falls to 5. 5 or below and stays
that way for a while. This causes holes to
form in the enamel, eventually exposing the
underlying dentin to the acidic environment.
Being only about 70% mineral, dentin is less
In the mouth, saliva has a pH ranging
from 6.2 to about 7. 4. It contains
chemical compounds, called buffers,
that resist pH change.
www.acs.org/chemmatters 10 ChemMatters | OCTOBER/NOVEMBER 2015
Left: Tooth decay, which is due to bacteria,
caused a hole, or cavity, to form in the tooth.
Right: A resin filling plugs the hole and prevents
the tooth from further decaying.
This X-ray image shows resin fillings (white) that
filled cavities in three top teeth and two