Fluorine is a pale, yellow-green gas that has a strong, sharp odor. It combines with hydrogen to make hydrogen fluoride, a colorless gas. Hydrogen fluoride dissolves in water to form hydrofluoric acid. Fluorine also combines with metals to make fluorides like sodium fluoride and calcium fluoride, both white solids. Sodium fluoride dissolves easily in water, but calcium fluoride doesn't.
Fluorine is used in rocket fuels, glass, enamel, and bricks.
Toothpaste and mouth rinses have fluorides added to prevent cavities. If drinking water supplies are low in fluoride, many communities add fluorides to help prevent cavities. Some skin medicines and cancer treatment drugs also contain fluorides. Fluorine forms salts with minerals in soil, and doesn't evaporate back into air as a gas. Fluorides in released to the air from volcanoes and industry are carried by wind and rain to nearby water, soil, and food sources.