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Answer:
First of all, let’s take a look at the structure of teeth
(see picture on below).
The surface of a tooth is surrounded by a layer called enamel. 95%
Of enamel is made from a substance called “hydroxyapatite”, which is
very hard and strong. There is “dentin” inside, which surrounds the
nerve in the centre.
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Let’s focus on hydroxyapatite next. This substance is very hard, but
it is easily attacked by acid. In fact, a tooth,
after being soaked in vinegar overnight, can be
easily cut by a knife.
Why is hydroxyapatite easily attacked by acid? The chemical formula
for this substance is Ca10(PO4)6(OH)2.
The following reaction occurs on the surface of a tooth when acid is
applied:
Ca10(PO4)6(OH)2
＋
8H+
↔
10Ca2+
＋
6(HPO4)2-
＋
2H2O
This equation means that the reaction will proceed toward the right
side by the addition of acid (H+ ion). Thus, the more acid
added, the more hydroxyapatite that is dissolved into
the saliva. This reaction
occurs at pH lower than 7. (This reaction is called
“decalcification” since the calcium in hydroxyapatite is dissolved.)
This reaction is also important in explaining tooth decay.
Bacteria in plaque decompose sugar and form acid. Hydroxyapatite on
the surface of teeth is dissolved by this acid. This is the main cause
of tooth decay.
Incidentally, squeezed lemon juice is a strong acidic solution (pH
≈ 2) because of the citric acid contained in lemon juice. It
could be true that teeth would be whitened temporarily by wiping with
squeezed lemon juice, because hydroxyapatite on the surface of teeth
is dissolved by the acid (H+) in lemon juice and then a new
surface appears. However, this is analogous to causing tooth decay on
purpose. Therefore, this course of action is NOT recommended.
Acknowledgement
Following experts provided us with several suggestions for this
answer. We really thank them for their help and kindness.
White-family dental clinic
Igarashi dental clinic
This article is translated by Chemistryquestion.com from the original article in Chemistryquestion.jp. Please let us know if you find any errors.
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