Caustic soda is one of the common names for sodium hydroxide (NaOH), which is also known as lye. Its common name derives from its chemical identity as a sodium hydrate and because it is caustic or corrosive. In pure form, caustic soda is a waxy, white solid. It readily absorbs water and forms aqueous solutions.Sodium Hydroxide (as NaOH) 99.5% minSodium Carbonate (as Na2CO3) 0.40% maxChlorides (as NaCl) 1000 ppm max.Silicates (as SiO2) 200 ppm max.Iron (as Fe) 20 ppm max.Copper (as Cu) 2 ppm max.