Both sodium hydroxide and calcium hydroxide are inorganic bases. Calcium hydroxide is traditionally called slaked lime, and sodium hydroxide is often called lye or caustic soda. Calcium hydroxide is a colorless crystal or white powder. Sodium hydroxide is available in semi-clear pellets or blanks or in a stock solution. Both have industrial and household applications.
Sodium hydroxide and calcium hydroxide have important differences. Sodium hydroxide has one hydroxyl group and calcium hydroxide has two hydroxyl groups. The Centers for Disease Control explains that when you mix sodium hydroxide with water or acid, the reaction is so exothermic that nearby combustible materials may ignite. Although calcium carbonate is equally reactive, a comparison of Material Safety Data Sheets for both chemicals shows that it is less hazardous.
Calcium hydroxide can be prepared by treating calcium oxide with water. Alternatively, it can be synthesized by mixing solutions of calcium chloride and sodium hydroxide.
Sodium hydroxide is industrially prepared using the chloralkali process, which involves the electrolysis of a sodium chloride solution. Chlorine and hydrogen are other products of this reaction.
Calcium hydroxide is used as a flocculant. Flocculants clarify solutions by removing small particles. It is also used to produce kraft paper. It is a common ingredient in building materials such as mortar, plaster and whitewash and in personal care depilatories.
Sodium hydroxide is found in oven and drain cleaners. Industrially, it is used in soap making, aluminium etching and cocoa processing. It is a common lab reagent.
While calcium hydroxide and sodium hydroxide are both corrosive, calcium hydroxide is less so. Both burn the skin and airways upon contact. Both are potentially lethal poisons. Get prompt medical care if you ingest either of these chemicals. Do not induce vomiting, because that will only expose more tissue to the chemical. Treat skin burns by flushing with water.