NEUTRALIZATION; SALTS 193 



Sodium carbonate + calcium hydroxide give 

 (soluble) (soluble) 



Sodium hydroxide + calcium carbonate. 



(soluble) (insoluble) 



The insoluble calcium carbonate settles, and the solu- 

 tion of sodium hydroxide can be poured off. When the 

 solution is evaporated, the sodium hydroxide remains as 

 a white solid. Potassium hydroxide is made in a similar 

 way. 



Both of these solids are very soluble in water; they even 

 attract water from the air, becoming wet, like some 

 candies. They are changed back to carbonates by the 

 carbon dioxide of the air (cf. 218). Their water solutions 

 are " slimy " to the touch (cf. 109), and act vigorously 

 upon the skin, the fats, and other organic matter. The 

 taste of a very dilute solution of these " caustic alkalies " 

 is bitter. Strong solutions destroy the mucous membrane 

 of the mouth (cf. 359) as a hot body would. Hence the 

 name " caustic, " meaning burning. 



220. Neutralization ; Salts. If caustic soda solution is 

 added drop by drop to hydrochloric acid containing lit- 

 mus, a point is found at which the litmus is neither red nor 

 blue, but has a lavender color. If blue or red litmus is put 

 into the solution, it is either not changed at all, or it be- 

 comes lavender. We say that the caustic soda has neu- 

 tralized the acid. 



We can neutralize the hydrochloric acid by sodium 

 carbonate also; but in this case there is carbon dioxide 

 given off. If the neutral solutions are now evaporated, 

 each will give a residue of cubical, white crystals. The 



