ACTION OF ACIDS WITH CARBONATES 



191 



painted upon copper with asphalt paint (Fig. 193), and the copper is 

 put into nitric acid, the part not covered by the asphalt is " etched " 

 by the acid. When the asphalt is removed, 

 the design stands out "in relief." 



217. Action of Acids with Carbon- 

 ates. -The action of acids with car- 

 bonates may also be used as a test 

 for acids. Marble (cf. 132) and 

 hydrochloric acid react with much FIG. 193. 



, i T i This design may be etched 



effervescence, because carbon dioxide n copper by dilute nitric 



acid. 



escapes as a gas (cf. 126). The 



other product is calcium chloride. It may be obtained as 



a white solid by the evaporation of its solution. 



The limestone of bones, oyster and clam shells, and 

 of coral is rapidly eaten out by acids, and only the animal 

 material is left. As a result, bones that are treated with 

 acids lose their stiffening. The large amount of acid in a 

 dog's stomach enables him to digest bone. 



Other carbonates react with acids as marble and limestone do. 

 Washing soda is sodium carbonate, which is made up of sodium, carbon, 

 and oxygen. Baking soda is sodium hydrogen carbonate, or sodium 

 bicarbonate (cf. 130). Both washing soda and baking soda effervesc^ 

 rapidly when an acid is added. The acid used may be tomato juice, 

 sour milk, or lemon juice, as well as a mineral acid (cf. 214). When 

 either of the two " sodas " is treated with hydrochloric acid, there is 

 formed, besides water and carbon dioxide, sodium chloride, or common 

 salt (cf. 108). Soda was formerly rare and expensive; but it is now 

 made, on an enormous scale, from common salt. 



Wood ashes contain potassium carbonate, or " potash." The potash 

 is obtained from the ashes by the use of water. Potash reacts with 

 hydrochloric acid as soda does; but it gives potassium chloride instead 

 of sodium chloride. 



