hydroxide [key], chemical compound that contains the hydroxyl (−OH) radical. The term refers especially to inorganic compounds. Organic compounds that have the hydroxyl radical as a functional group are called alcohols; the hydroxyl radical is also present in the carboxyl group of organic acids. Most metal hydroxides are bases—they form solutions that have an excess of OH− ions and a pH greater than 7, neutralize acids, and change the color of litmus from red to blue. Alkali metal hydroxides such as sodium hydroxide are strong bases and are very soluble in water; alkaline–earth metal hydroxides such as calcium hydroxide (slaked lime) are much less soluble in water and are not as strongly basic. Magnesium hydroxide (milk of magnesia) is only slightly basic. Some hydroxides (e.g., aluminum hydroxide) exhibit amphoterism, having either acidic or basic properties depending on the reaction in which they are involved. The hydroxides of some nonmetallic elements are acidic; the hydroxide of sulfur, S(OH)6, spontaneously loses two molecules of water to form sulfuric acid, H2SO4. Ammonium hydroxide, NH4OH, is a weak base known only in the solution that is formed when the gas ammonia, NH3, dissolves in water.
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