SOME COMPOUNDS OF COMMON ELEMENTS 163 



183. Carbonates. Carbonates are salts containing va- 

 rious metals combined with carbon and oxygen. Sodium 

 carbonate is the common washing soda, and calcium car- 

 bonate is the compound of which limestone and marble 

 are composed. Sodium and potassium carbonates are 

 much used in the preparation of soaps and washing 

 powders, because they change fats and grease to soluble 

 substances. 



Carbonates are valued because by simple treatment car- 

 bon dioxide gas can be procured from them. Any car- 



Battery i 



FIG. 80. A SILVER PLATING BATH 



The tank contains a solution of some compound of silver. One wire 

 from the battery connects with a bar of silver, and the other wire with a 

 spoon which is to be plated. 1. Starting with the positive electrode of the 

 battery, name all the parts of the circuit. 2. When a current is passing, 

 molecules of silver leave the solution and cling to the spoon. Their places 

 in the solution are taken by molecules from the bar of silver. Which 

 way, then, does the current go in the plating solution? 



bonate decomposes on the addition of even a weak acid, 

 giving off carbon dioxide. 



Sodium bicarbonate is used in the preparation of some 

 foods. Baking powder consists of a dry carbonate mixed 

 with a weak acid in powder form. As long as both sub- 

 stances are dry, they do not affect each other; but as soon 

 as they are dissolved in water or milk, they act on each 



