ACIDS, ALKALIES, AND SALTS 



137 



changes to red, the solution is an acid. If the paper remains 

 blue, the solution is not acid. Acids have different powers 

 and uses. While some are health- 

 ful and are used for foods, others 

 are poisonous. Acids are used 

 very commonly in industry for 

 dissolving metals (Fig. 67). 



147. Mineral and Organic 

 Acids. There are two kinds of 

 acids organic and mineral. Or- 

 ganic acids are those, such as car- 

 bolic acid, oxalic acid, etc., which 

 contain the element carbon in 

 their composition. Mineral acids 

 are those composed of any of the 

 other elements, such as hydrochloric 

 acid, nitric acid, and sulphuric acid, 

 and are used principally in the 

 trades and industries. 



Nitric acid is largely used in the 

 manufacture of explosives, and hydrochloric acid as a " pick- 

 ling" liquor for cleaning metals. When nitric acid is added 

 to some metals, it acts very quickly, and gives off reddish 

 brown fumes that are suffocating in their effect. 



The change that takes place is represented by the following 

 equation : 



3 Cu + 8 HNO 3 3 Cu (NO 3 ) 2 + 4 H 2 O + 2 NO 



Copper Nitric Copper Water Nitric 



Acid Nitrate Oxide 



FIG. 67. One Step in the 

 Analysis of Iron. The 

 iron is carefully weighed 

 and then dissolved in acid 

 over a lamp. 



The copper nitrate and water remain and the gas (nitric oxide) 



