CHAPTER XII 

 ACIDS, ALKALIES, AND SALTS 



143. Classes of Compounds. Compounds may be di- 

 vided roughly into four classes of substances: water, acids, 

 bases or alkalies, and salts. 



144. Properties of Water. Pure water is the commonest 

 compound that exists. The water we use comes to us either 

 in the form of rain or of melted snow from the mountains. 

 Part of it trickles or percolates through the ground and 

 dissolves any soluble material or gases with which it comes 

 in contact. When the water has passed into the ground and 

 comes in contact with limestone and magnesium compounds, 

 some of the substances are dissolved and the water becomes 

 hard. This kind of water appears when an artesian well is 

 drilled. Water that flows over the surface of the earth 

 contains suspended matter or dirt and is generally called 

 soft water. 



Thus the distinction between hard and soft water depends 

 upon the substances which they carry, and especially upon 

 their chemical action. In soft water, soap readily lathers 

 and the suds thus formed exert a rapid cleansing action. In 

 hard water, soap lathers only with difficulty and often will 

 not lather satisfactorily at all, because of the formation of 

 lime soap, which is insoluble. 



If hard water is boiled the hardness often disappears, and 

 soap then acts as in soft water, but in some cases boiling 



135 



