SOME COMPOUNDS OF COMMON ELEMENTS 161 



red. Litmus paper is paper colored by a dye from a certain 

 plant. This dye changes from blue color to red when in 

 contact with even the vapor or fumes of an acid. 



The most common acids used in the industries are hydro- 

 chloric acid (also called muriatic), sulphuric (called oil of 

 vitriol), and nitric acid. In full strength these acids are 

 injurious to the flesh and the clothing. They may be diluted 

 with water and in very dilute form are harmless. Sulphuric 

 acid is used in making most of the other acids. 



Other acids, such as oxalic, tartaric, and citric, are found in 

 dilute forms in the juices of fruits and leaves of plants. Acetic 

 acid is formed when cider " works" or ferments, and it is the 

 acid which gives vinegar its sour taste. 



180. Bases. In general, bases are opposite to acids in 

 their properties. Their solutions turn red litmus paper blue. 

 Ammonium hydroxide (called ammonia water), sodium 

 hydroxide (called caustic soda), and calcium hydroxide 

 (called slaked lime) are bases. 



Most burns and stings of insects are painful because of 

 an acid formed by the burn or injected in the stinging. 

 The application of certain basic materials, such as soap, 

 cooking soda, or lime water, counteracts the acid and thus 

 affords relief. 



181. Salts. If an acid and a base are put together in 

 the right proportions, the characteristic properties of both 

 disappear. Since the product is neither an acid nor a base, 

 it is said to be neutral. If the water is evaporated from 

 the resulting solution, there remains a crystalline or powdered 

 solid called a salt, which is not like either the acid or the 

 base. 



Nearly all minerals are salts. Most of these salts found in 

 the rocks are only slightly soluble. The more soluble salts 

 are dissolved by the water which is continually washing over 

 and through the earth's crust, and are carried finally into 

 the ocean. Since this has been going on through all the ages, 



