pH, Solubility, and Tooth Decay
Tooth decay has plagued humans for centuries. Although its cause is fairly well understood, total prevention of tooth decay is still not possible.
Teeth are protected by a hard enamel layer about 2 mm thick that is composed of a mineral called hydroxyapatite, Ca5(PO4)3OH. When it dissolves (a process called demineralization), the ions go into solution in the saliva:
Ca5(PO4)3OH (s) -> 5Ca2+ (aq) + 3PO43- (aq) + OH- (aq)
Because phosphates of alkaline earth metals such as calcium are insoluble, this reaction processds only to a small extent. The reverse process, called re-mineralization, is the body’s natural defense against tooth decay:
5Ca2+ (aq) + 3PO43- (aq) + OH- (aq) -> Ca5(PO4)3OH (s)
In children, the growth of the enamel layer (mineralization) occurs faster than demineralization; in adults, demineralization and remineralization take place at roughly the same rate.
After a meal, bacteria in the mouth break down some food to produce organic acids such as acetic acid and lactic acid, CH3CH(OH)COOH. Acid production is greatest from food with high sugar content, such as candies, ice cream, and sugary beverages. The decrease in pH results in the removal of OH- ions promoting demineralization. Once the protective enamel layer is weakened, tooth decay starts.
H+ (aq) + OH- (aq) -> H2O (l)
The best way to fight tooth decay is to eat a diet low in sugar and always brush one’s teeth immediately after eating. Most toothpastes contain a fluoride compound such as NaF or SnF2 that also helps to reduce tooth decay. The F- ions from these compounds replace some of the OH- ions during the remineralization process:
5Ca2+ (aq) + 3PO43- (aq) + F- (aq) -> Ca5(PO4)3F (s)
Because F- is a weaker base than OH-, the modified enamel, called fluorapatite, is more resistant to acid.
plague: gây hại, tai họa
prevention: sự phòng ngừa, sự cản trở
enamel: men (răng), lớp men
demineralization: sự khử khoáng
saliva: nước bọt
extent: phạm vi
reverse: ngược lại
sugary beverage: đồ uống có đường
immediately: ngay, ngay lập tức, tức thì
resistant: bền, sức chịu đựng, đề kháng