308 ELEMENTARY GENERAL SCIENCE CHAP. 



be remembered that it only exists in its salts and has no inde- 

 pendent existence. 



EXPT. 310. Heat an ammonium salt with caustic soda or 

 lime, or merely well mix the two in a mortar with the addition 

 of a little water. Observe by the smell that ammonia is 

 produced. 



The gas ammonia, then, is very readily driven off from am- 

 monium salts by the action of caustic soda, &c., this action is 

 readily understood from the equation 



NH 4 C1 + NaOH = NaCl + NH 4 OH. 



The latter product is an aqueous solution of ammonia, so the 

 equation may also be written thus : 



NH 4 C1 + NaOH = NaCl + NH 3 + H 2 O. 



CHIEF POINTS OF CHAPTER XXII. 



Acids are compounds containing hydrogen which is capable of being 

 replaced by a metal with the formation of salts, e.g., hydrochloric, 

 sulphuric and nitric acids. 



Salts are compounds which result from acids by the replacement 

 of .their hydrogen by a metal. 



Preparation of Salts. Salts may be prepared either by the direct 

 action of an acid on a metal, or by the action of an acid on an oxide. 

 In the former case hydrogen is sometimes liberated as such, while in 

 other cases it reacts with some of the acid, forming secondarj 7 products. 

 When an oxide is used water is formed in place of hydrogen. 



Examples of Salts. The following salts among others are prepared 

 by means of the experiments described, copper nitrate, the sulphate 

 of magnesium, which is also known as Epsom salts, the chloride and 

 nitrate of magnesium, and similarly the sulphate, chloride, and 

 nitrate of calcium. 



Reactions of Hydroxides with Acids. Salts are also formed by the 

 action of acids upon the hydroxides, such as, sodium hydroxide, 

 NaOH, or, as it is also called, caustic soda. Since oxides and 

 hydroxides neutralise acids to form salts they are both included in 

 the class of compounds called Bases. 



Bases are compounds, known as oxides or hydroxides of metals, 

 which are capable of neutralising acids with the formation of salts 

 and water. 



Alkalis are those bases which are soluble in water. They are 

 characterised by an alkaline reaction. Sodium hydroxide, potassium 

 hydroxide, &c. , are examples. 



