We have two sets of teeth during our life time. The initial 20 milk teeth are replaced by 32 permanent teeth. These are bony cells that grow out of the gum in the mouth. Each tooth consists of a crown and one to three roots depending on its type and position. The crown is that part which appears above the surface of the gum, while the root is contained in the socket in the jaws. The bulk of the tooth consists of a hard yellow material known as dentine. The exposed surfaces of the crown are covered by the dense white enamel which is the hardest tissue in our body. Its molecules are Iived up in patterns called crystals. The sort middle part of a tooth is called the pulp and is filled with nerves and blood vessels.
Fluoride is a chemical substance that can make enamel even stronger. If we brush our teeth with fluoride tooth paste, molecules of fluoride arrange themselves in the crystalline pattern, replacing other molecules. Since the fluoride molecules fit more tightly than the others, they make the enamel harder to break. The harder the enamel, the fewer the cavities. In many cities today fluoride is added to the water supply. Fluoride in the crystals strengthens the teeth by making the enamel hard.
Fluorides are compounds of fluorine and another element or group of elements. Most of the fluorides are salts. They are produced by the reaction of hydrofluoric acid with a metal oxide.