,58 PRINCIPLES OF CHEMISTRY. 



A 



/ Synthesis is that method of research by which bodies are made to 



vunite to produce substances more complex. 



Analytical and synthetical methods, or reactions, frequently blend 

 into one another. This means : A reaction made with the intention 

 of recognizing a substance may at the same time produce some com- 

 pound of interest from a synthetical point of view. 



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. 



Acids are characterized by the following properties : 



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



2. They change the color of many organic substances, for instance 

 of litmus, from blue to red. 



3. They contain hydrogen, which can be replaced by metals, the 

 compound thus formed being a salt. 



/ According to the number of hydrogen atoms replaceable by metals, 

 / we distinguish monobasic, dibasic, and tribasic acids. Hydrochloric 

 \ acid, HC1, is a monobasic, sulphuric acid, H 2 SO 4 , is a dibasic, phos- 

 phoric acid, H 3 PO 4 , is a tribasic acid. 



Bases or basic substances show properties which are chemically 

 opposite to those of acids. These properties are : 



1. They have (when soluble in water) the taste of lye, or an alka- 

 line taste. 



2. They have (when soluble in water) an alkaline reaction, i. e. y 

 they restore the color of organic substances when previously changed 

 by a9ids, for instance that of litmus, from red to blue. 



3. When acted upon by acids, they form salts. For instance: 

 Potassium hydroxide is a base ; when brought in contact with hy- 

 drochloric acid it forms water and the salt potassium chloride : 



KOH + HC1 = H 2 + KC1. 



Neutral substances. All substances having neither acid nor basic 

 properties are neutral. Water, for instance, is a neutral substance, 

 having no acid or alkaline taste, and no action on red or blue litmus. 

 Many neutral substances, to some extent even water, appear to possess 



