Sodium hydroxide, whose chemical formula is NaOH, commonly known as caustic soda, caustic soda and caustic soda, is a strong caustic with strong corrosiveness, generally in the form of flake or block, which is easy to dissolve in water (release heat when dissolving in water) and form an alkaline solution. In addition, it has deliquescence, which is easy to absorb water vapor (deliquescence) and carbon dioxide (metamorphism) in the air. Hydrochloric acid can be added to test whether it is metamorphism.