Precipitation—This refers to the chemist’s precipitation, not the meteorologist’s term. Precipitation, the formation of a solid, certainly applies to the process of mineralization, the formation of tooth enamel via the deposition of hydroxyapatite on the surface of the tooth.
Dissolving and dissolution—The dissolving of a solid into solution occurs in demineralization, the process whereby hydroxyapatite dissolves off the tooth’s surface due to increased acidity in the mouth.
Equilibrium—The equilibria discussed in this article, acid-base interactions in the mouth and those in the body, are great examples of chemical equilibria at work in nature.
Le Châtelier’s Principle and factors affecting equilibrium—The article shows how eating food and exercise can be stresses to equilibrium systems within the body.
Acid-Base chemistry—Although the article focuses primarily on acids, specifically lactic and carbonic acids, this information is a nice segue into the effects of bases on these equilibrium systems, and it offers the teacher a chance to show the interrelatedness of acids and bases in chemical reactions.
pH—The article reinforces the importance of pH in our study of acids and bases.
pH, Kas, pKas—The acidity of the mouth with/without food, and its role in enamel/dentin decay could be used in a class discussion of acid strength.
Minerals—Minerals are usually thought of as materials that are mined from or just under the Earth’s surface, but we frequently forget that they can also be found in living organisms, as in teeth and bones in animals.
Fluorine/fluoride chemistry—Students don’t get many opportunities to discuss fluorine’s chemical reactivity. The application of fluoride treatments in the prevention of tooth decay gives them one example.
Adsorption—Fluoride used to prevent tooth decay is adsorbed onto the tooth surface, protecting the enamel from acids produced in the mouth by bacteria, or ingested from outside food/drink sources.
Biochemistry—Almost all of biochemistry involves rather complex organic chemistry; this article provides a few simple, easy-to-understand (for first year chemistry students) inorganic reactions that affect myriad biochemical reactions in the body.
Possible Student Misconceptions
(to aid teacher in addressing misconceptions)
“Chemical reactions are irreversible—they only go one way, ‘left to right’.”Early in the academic year, all equations representing chemical reactions are written for students with the arrow pointing exclusively to the right, giving the student the illusion that reactions only go one way. It isn’t until we reach the topic of equilibrium that most students finally see that chemical reactions can be reversed, or can go in both directions simultaneously.
“It doesn’t matter when I eat sweet snacks, they’re always bad for my teeth.” Current research in tooth decay shows that sweets eaten with a meal are less likely to cause tooth decay than sweets eaten by themselves between meals, primarily because the residual sugars that feed the bacteria that reside in the mouth are washed away with the rest of the food and beverage during a meal. And saliva produced from the foods eaten during the meal helps buffer the effects of the acids produced. Also, you’re more likely to brush your teeth after meals than after between-meal snacks.
“If I use fluoride toothpaste, I won’t get any cavities.”While fluoride treatment does provide another level of protection for your teeth, it does not prevent food materials from getting stuck to teeth. Plaque can still build up on tooth surfaces, resulting in bacterial decay of the enamel that eventually will lead to cavities. Good oral hygiene is still critical—see next misconception.
“If I use a fluoride mouth rinse and my dentist puts fluoride stuff on my teeth, I don’t have to brush them.”While both these procedures will reduce tooth decay, nothing works as well as brushing after meals (and snacks). The fluoride in mouth rinses and dental treatments makes tooth enamel more resistant to acids produced by bacteria as they digest sugars. (see “More on fluoride treatments”, in the Background Information section, above) Fluoride does not, however, make teeth impervious to those acids. So it’s critical that you remove the foodstuff and bacteria by brushing and flossing regularly.
“Mercury in amalgam fillings is dangerous and dentists should stop using them.”There is much controversy today about the use of mercury amalgam fillings, but there is little scientific evidence that the mercury leaches out of the amalgams into the bloodstream, and the American Dental Association and the Federal Food and Drug Administration have not banned their use.
“If I have mercury amalgam fillings, I should go to my dentist right away and have them removed.”If your fillings are in good condition and there is no decay beneath the filling, FDA does not recommend that you have your amalgam fillings removed or replaced. Removing sound amalgam fillings results in unnecessary loss of healthy tooth structure, and exposes you to additional mercury vapor released during the removal process.” (http://www.fda.gov/MedicalDevices/ProductsandMedicalProcedures/DentalProducts/DentalAmalgam/ucm171094.htm)
(answers to questions students might ask in class)
“How do acids attack tooth enamel?” Essentially, the H+ ion from the organic acids produced by bacteria digesting food reacts with the hydroxyl group (OH–) from hydroxyapatite, as well as with the phosphate groups (PO43-). See “More on tooth decay” in the Background Information section, above.
“What is it about sweets that makes them so bad for our oral health?”Sweets contain sugars, and sugars are the main food source for bacterial metabolic processes. Bacteria metabolizing these sugars results produce organic acids, which are responsible for demineralizing tooth enamel.
“What is the other 4% of the enamel layer of teeth?”The remaining 4% of tooth enamel is organic material and water.
“Is carbonic acid a ‘strong’ acid? Is that why it’s bad for our teeth?”Carbonic acid is not a particularly strong acid, as acids go; in fact, it is a rather weak acid, compared to laboratory acids like sulfuric and hydrochloric acids. But even weaker acids can, over time, affect the dissolution of hydroxyapatite from tooth enamel. Acidic foods, such as citrus fruit and spaghetti sauce, also contain weak acids. Actually, strong acids cannot be used in foods, as they would react adversely with organic material in the body. Think battery acid, or concentrated hydrochloric acid from the lab.
“Is the carbonic acid involved in the chemical equilibrium equations in this article the same carbonic acid as the stuff found in sodas (or ‘pop’)?” Yup. See following question.
“Why is drinking soda (pop) so bad for your teeth?”See previous question. But it’s not only (or even primarily) the carbonic acid found “naturally” in carbonated beverages that is responsible for the acidic erosion of tooth enamel, rather it is the extra, stronger acids added to sodas—phosphoric acid and/or citric acid that are added to increase the “sharp” or tingly taste in sodas. In addition, sodas typically contain high fructose corn syrup (HFCS), a simple sugar “loved” by bacteria, enabling them to produce more organic acids that attack enamel. This bacteria food source and the additional acid provided by the dissolved carbon dioxide in soda and the other added inorganic acids, plus all the organic acids produced by the ebullient bacteria, provides a double whammy for your teeth. But don’t think for a moment that diet sodas are off the hook. Although they do not contain HFCSs, they still contain phosphoric and/or citric acids that will erode tooth enamel over time. So, although they’re better than sodas in terms of enamel erosion, they still contribute to the process.
“Does fluoride really help fight cavities?” Many studies have shown the positive effect fluoride has on tooth decay. Chemically, it greatly aids the remineralization of tooth enamel.
“Is it true that fluoride in drinking water will stain your teeth?” Yes and no. It is true that at concentrations higher than 1 ppm fluoride in drinking water can definitely produce stained, or what are referred to as “mottled” teeth. In fact, the discovery that fluoride in drinking water can reduce the incidence of cavities in teeth came about when early in this century it was noted that people who lived around Colorado Springs had a very high incidence of stained or “mottled” teeth, but at the same time had a very low incidence of dental cavities. Both effects were eventually connected to high concentrations of fluoride in the local drinking water. But at concentrations below 1 ppm this staining does not occur to any significant degree.
“Are mercury amalgams dangerous?”This question has been argued over for decades, and the debate continues even today. See “More on treating tooth decay” in the Background Section above. There are definitely drawbacks to using amalgams—problems obtaining a useful 3-dimensional panoramic x-ray picture, the need to cut away extra, healthy tooth to accommodate the shaping of the filling to ensure an immobile fit, and the need for proper disposal due to the mercury content—all as mentioned in the article. In addition, many people believe that some of the mercury itself can leach out of the amalgam and travel through the bloodstream and cause nerve damage within the body. Studies to-date have not supported that belief, and the American Dental Association and other professional groups have not seen the need to ban the use of amalgams, although composite resin fillings are certainly becoming more prevalent and popular.
“If I have mercury amalgam fillings, should I ask my dentist to remove them and replace them with composite resin fillings?” “If your fillings are in good condition and there is no decay beneath the filling, FDA [U.S. Food and Drug Administration] does not recommend that you have your amalgam fillings removed or replaced. Removing sound amalgam fillings results in unnecessary loss of healthy tooth structure, and exposes you to additional mercury vapor released during the removal process.” (U.S. Food and Drug Administration: http://www.fda.gov/MedicalDevices/ProductsandMedicalProcedures/DentalProducts/DentalAmalgam/ucm171094.htm)