is any of a broad class of compounds, including alkalis, that react with acids to form salts, plus water. Also known as hydroxides. Hydroxides ionize in solution to form hydroxyl ions (OH-); the higher the concentration of these ions, the stronger the base. Bases are used extensively in petroleum refining in caustic washing of process streams to remove acidic impurities, and are components in certain additives that neutralize weak acids formed during oxidation.