192 ACIDS, ALKALIES, AND CLEANING 



218. Alkalies, or Bases. Bases are substances having 

 properties quite different from those of acids. They turn 

 litmus which has been colored red by acids back to the 

 blue color. All substances that do this are said to have a 

 " basic " reaction. The strongest (most active) bases 

 are called alkalies; hence a basic reaction is also called 

 an alkaline reaction. 



The two strongest bases are commonly known as lye. 

 Better names are sodium hydroxide (caustic soda) and 

 potassium hydroxide (caustic potash). As the chemical 

 names show, these substances are composed of hydrogen 

 and oxygen, combined in the one case with the metal 

 sodium, and in the other with the metal potassium (cf. 

 109). Other common bases are ammonium hydroxide 

 ("ammonia water"; cf. 112) and calcium hydroxide 

 (slaked lime). Calcium hydroxide is the cheapest base. 

 It is made by adding water to calcium oxide (quicklime). 

 Quicklime is made by heating limestone (cf. 132). 



Bases cannot be kept if exposed to the air; for they unite with its 

 carbon dioxide (carbonic acid; cf 126) to form the carbonates. Lime 

 that is " air slaked " is largely calcium carbonate. Solutions of potas- 

 sium and sodium carbonates have an alkaline reaction, and behave 

 like weak alkalies in other respects, because they react with water 

 to form small amounts of the hydroxides. 



219. Caustic Soda and Caustic Potash. When sodium 

 carbonate (soda) solution is mixed with " milk of lime," 

 which is slaked lime " suspended " in lime water (cf. 132), 

 a chemical change occurs, and calcium carbonate is formed 

 as an insoluble powder. The sodium hydroxide (caustic 

 soda) remains in solution. 



