|MadSci Network: Medicine|
As you may have learned already from your dentist, cavities (holes in your teeth) are caused by the bacteria that live in your mouth. As you eat food, some small pieces get left in and around your teeth. The food left behind is eaten by the bacteria which, unfortunately, produce small amounts of acid as they eat. Over time, the constant exposure of your teeth to this weak acid is enough to create the holes in your enamel (the hard outer layer of teeth), which the dentist then fills to prevent further problems.
The reason your teeth get holes when exposed to acid is that the calcium in enamel (as in bones), which gives it its strength, is used as a compound called hydroxyapatite (hi-drock-see-a-pat-tight), which dissolves in weak acids. When you put acid on your teeth, it slowly dissolves the hydroxyapatite out of your teeth, so that all that is left is a soft, rubbery coat which is easily damaged, or eaten by the bacteria. When you use toothpaste (or tapwater) with fluoride, the fluoride atoms react chemically with the calcium, replacing the hydroxy molecules to form fluoroapatite (floor-oh-a-pat- tight), which does not dissolve in acid. So fluoridating your teeth doesn't actually make them stronger, but it does make them resistant to acid, so that the bacteria can't dissolve them and you won't get as many cavities.
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