Sodium hydroxide, also known as caustic soda, lye, or sodium hydrate, is an inorganic compound with the chemical formula NaOH and a relative molecular mass of 39.9970.

Sodium hydroxide is highly alkaline and extremely corrosive. It can be used as an acid neutralizer, complex masking agent, precipitating agent, precipitation masking agent, color developing agent, saponification agent, peeling agent, detergent, etc., with very wide applications.