54 PROPERTIES AND MAKE-UP OF MATTER 



oil and water their failure to combine into a third substance 

 is even more noticeable. After a little while the water will 

 be found at the bottom of the vessel and the oil, which is 

 lighter, will float on top. A chemical compound is very 

 different from such mixtures, as we 

 learned in the case of water and of 

 iron sulphide. 



Acids, Bases, and Salts. The 

 most important chemical compounds 

 for us to consider are acids, bases, 

 and salts. Acids of various kinds 

 exist in apples, grapes, rhubarb, 

 buttermilk, vinegar, lemons, oranges, 

 and other familiar substances. 



A small amount of very dilute 

 RUSTING OF IRON hydrochloric acid is formed in the 

 stomach of man and of some other 



animals and helps in the process of digestion. Hydrochloric 

 acid, sulphuric acid, and nitric acid are much used in the 

 laboratories and in various industries. 



Many acids are liquid; and dilute solutions (little acid 

 in much water) of all common acids taste sour. Acids 

 turn blue litmus paper to red. Litmus paper is paper which 

 has been especially prepared by treating it with a vegetable 

 substance called litmus, obtained from a low order of plants 

 called lichens. Strong acids may cause great injury to 

 cloth, paper, wood, or the flesh of animals'. 



It is important that we should become acquainted with 

 another class of compounds called bases that are in some 

 ways just the opposites of acids. Most bases are in the 

 form of solids; and dilute solutions of almost all the bases 



