xxn ACIDS, BASES, AND SALTS 301 



the action is readily proved to take place by the loss of its 

 characteristic properties by the acid. Owing to its insolubility it 

 is also formed as a precipitate, when sulphuric acid is added to 

 a fairly strong solution of calcium chloride or calcium nitrate. It 

 is largely used technically, being the substance known as gypsum 

 or plash'r of Paris, which possesses the property of setting to a 

 hard mass when mixed with water. The formation of these 

 salts may be represented by the equations : 



CaO + 2HNO 3 = Ca(N 3 ) 2 + H 2 O. 

 CaO + 2HC1 = CaCl 2 + H 2 O. 

 CaO + H 2 SO 4 = CaSO 4 + H 2 O. 



Reactions of Hydroxides with Acids. It has been shown 

 that caustic soda and caustic potash are also capable of forming 

 salts. These compounds are, however, not oxides, but are 

 known as hydroxides, i.e., compounds resulting from the dis- 

 placement of only half the hydrogen in water, or compounds 

 containing one or more OH groups. Caustic soda is thus sodium 

 hydroxide, NaOH, and caustic potash is potassium hydroxide, 1 

 KOH. The latter compound may be prepared by placing the 

 metal potassium on water. In this case the action is far more 

 energetic than is the case with sodium, and the metal appears to 

 burn on the surface of the water with a violet flame. In reality 

 it is the liberated hydrogen which burns, the violet colour being 

 due to vapours of potassium. 



EXPT. 304. Place a small piece of potassium on water, and 

 afterwards evaporate the liquid to dryness. Examine the 

 solid produced, and compare it with caustic soda. 



Caustic soda and caustic potash will be found to resemble one 

 another closely. They are both white solids, with a peculiar 

 soapy feel, both turn red litmus blue, and both absorb mois- 

 ture and carbon dioxide very readily from the air. 



EXPT. 305. Make solutions of caustic potash and caustic 

 soda, and neutralise both by (1) sulphuric acid, (2) nitric 

 acid, (3) hydrochloric acid. If care be taken the solution 

 may be made almost perfectly neutral, i.e., neither acid nor 

 alkaline. Evaporate the solutions so obtained and allow them 

 to crystallise. 



i These compounds are also called sodium hydrate and potassium hydrate, but 

 the term hydroxide is preferable. 



