Sodium hydroxide, also known as caustic soda, or lye, is an inorganic compound with the chemical formula NaOH (also written as NaHO) a white solid, and is a highly caustic metallic base and alkali salt. It is available in pellets, flakes, granules, and as a 50% saturated solution.

Sodium hydroxide is soluble in water, ethanol and methanol. This alkali is deliquescent and readily absorbs moisture and carbon dioxide in air.