Sodium Hydroxide by Causticizing Soda Ash with Slaked Lime

Before electrolysis took over in the 20th century, sodium hydroxide (caustic soda, lye) was made by "causticizing": treating sodium carbonate with slaked lime. Ca(OH)₂ + Na₂CO₃ → CaCO₃ + 2 NaOH. The reaction works because sodium hydroxide dissolves in water while calcium carbonate does not, so the carbonate falls out and the lye stays in solution. Soap makers did this first: a late 13th-century Yemeni book has water passed again and again through plant-ash alkali and quicklime. The soda ash later came from the Leblanc process and then the Solvay process. Evaporating the water from the solution gives solid sodium hydroxide.