160 JUNIOR GRADE SCIENCE 



or lime (calcium oxide). These facts may be collected together in the 

 form of a definition of an acid. Thus : 



An acid is a chemical compound with a sour taste, which usually has 

 the power of turning- blue litmus red. It always contains hydrogen, which 

 can be replaced by a metal, either directly, or by the action of an oxide, or 

 hydroxide, of a metal upon it. 



Bases and salts. Another class of compounds, of which sodium 

 hydrate and lime may be taken as typical examples, all possess pro- 

 perties of an opposite character to those which distinguish acids. They 

 all have the power of destroying, or neutralising, the properties of an 

 acid. These compounds are called bases. Some, such as caustic soda 

 and caustic potash, are soluble in water, and are called alkalies. A 

 solution of ammonia in water behaves like the two soluble bases 

 mentioned, and is also classed with the alkalies. 



Bases are always oxides or compounds of a metal with oxygen and 

 hydrogen, known as hydrates, or, better, hydroxides. When added to 

 an acid, the metal of the base replaces the hydrogen of the acid forming 

 a salt. This is a fact of great importance. We may write it in the 

 form of an equation : 



A BASE + An ACID = A SALT + WATER. 



This equation provides a definition of a base. It may be stated that : 



A base is an oxide or a hydroxide of a metal, capable of neutralising an 

 acid with the formation of a salt and water. 



The equation, too, also furnishes a convenient definition of a salt, 

 for, added to the knowledge previously obtained, it enables the state- 

 ment to be made, that : 



A salt is a chemical compound formed by acting upon an acid with a 

 base, in this way replacing the hydrogen of the acid with the metal. 



Names of salts. Before chemistry became the systematised study 

 which it is at present, it was the custom to name a salt after its dis- 

 coverer, or a name was sometimes given indicating some characteristic 

 property of it, or again the source from which it was obtained. The 

 system now adopted is to name salts in such a way as to show their 

 composition, or the metal and acid from which they are derived. Salts 

 obtained from sulphuric acid are named sulphates, those from hydro- 

 chloric chlorides, and those from nitric acid nitrates. For example, 

 the salt obtained in Expt. 78 i., which was recognised as common salt, 

 is called sodium chloride. The salt produced by the action of sul- 

 phuric acid on magnesium oxide (Epsom salts) is called magnesium 

 sulphate. 



