CHEMICAL EQUATIONS, TYPES OF CHEMICAL CHANGE, ETC. 117 



Examples of complete action because of the removal of one of the products 

 as a gas are: Action of any acid on any carbonate, whereby carbon dioxide 

 gas is liberated ; action of caustic alkalies, lime or magnesia, on ammonium 

 salts, whereby ammonia gas is liberated. 



Acids. The many compounds formed by the union of elements 

 are so various in their nature, that no system of classification pro- 

 posed up to the present time can be called perfect. There are, how- 

 ever, a few groups or classes of compounds, the properties of which 

 are so well marked, that a substance belonging to either of them may 

 be easily recognized. These groups are the acids, bases, and neutral 

 substances. 



The term acid is applied to those compounds of hydrogen with an 

 electro-negative element or group of elements which are character- 

 ized by the following properties : 



1. The hydrogen present is replaceable by metals, the compound 

 thus formed being a salt. 



2. They change the color of many organic substances. Thus, 

 litmus, a coloring-matter obtained from certain lichens, is changed 

 from blue to red. 



3. They have (when soluble in water) usually an acid or sour taste. 



The great majority of acids are the result of union between water and the 

 oxides of those elements which are devoid of characteristic metallic properties. 

 We might, therefore, classify non-metallic elements as acid-forming elements. 

 There are a few exceptional metals which form a series of oxides, some of 

 which, when united with water, give acids; for example, chromic acid, H 2 CrO 4 , 

 permanganic acid, HMnO 4 . The formation of acids from oxides is shown by 

 the following equations : 



SO 3 -f H 2 = H 2 SO 4 . 



Sulphur Sulphuric 



trioxide. acid. 



P 2 5 -f 3H 2 O : 2H 3 P0 4 . 



Phosphorus Phosphoric 



pentoxide. acid. 



C0 2 + H 2 O = H 2 C0 3 . 



Carbon Carbonic 



dioxide. acid. 



Evidently in the acids containing oxygen, often called oxyacids, the hydrogen 

 is derived from the water molecules with which the acidic oxides unite. Those 

 oxides which unite with water to give acids are called acidic oxides or acid 

 anhydrides. As was said before, the great majority of acidic oxides are derived 

 from the non-metals, but there are some oxides of the non-metals which do not 

 form acids, for example, carbon monoxide, CO, and nitrogen monoxide, N 7 O. 



A few acids contain no oxygen, and these are sometimes called hydracids. 

 They have no corresponding oxides and are combinations of hydrogen with 

 non-metallic elements, or groups of elements called radicals. The principal 

 ones are hvdrochloric acid, HC1, hydrobromic acid, HBr, hydriodic acid, HI, 



