CARBON, SILICON, BORON. 183 



Carbonic acid is a dibasic, extremely weak acid, the salts of which are 

 known as carbonates. Many of these carbonates (calcium carbonate, 

 for instance) are abundantly found in nature. Only the alkali car- 

 bonates and bicarbonates are soluble in water. Acid carbonates of 

 some other metals, such as magnesium, calcium, zinc, iron, are also 

 slightly soluble in water, but these do not exist in the dry state. The 

 bicarbonates when heated to about 100 C. give up carbon dioxide 

 and form carbonates : 



2NaHCO 3 = Na 2 CO 3 -f H 2 O -f CO^ 

 At high temperatures only alkali carbonates are not decomposed. 



Tests. Since nearly all carbonates are insoluble in water, these 

 are formed as precipitates whenever a solution of an alkali carbonate 

 (those of potassium, sodium, or ammonium) is added to a solution of 

 a salt of any other metal. This is a corroborative test for a soluble 

 carbonate, but the best and decisive test for all carbonates and car- 

 bonic acid is found in Experiment 12, namely, the liberation of 

 carbon dioxide and its action on lime-water. 



Carbon monoxide, carbonic oxide, CO. While carbon, as a 

 general rule, is quadrivalent, in this compound it exerts a valence of 

 2. Carbon monoxide is a colorless, odorless, tasteless, neutral gas, 

 almost insoluble in water ; it burns with a pale-blue flame, forming 

 carbon dioxide ; it is very poisonous when inhaled, forming with the 

 coloring matter of the blood a compound which prevents the absorp- 

 tion of oxygen. Carbon monoxide is formed when carbon dioxide is 

 passed over red-hot coal : 



C0 2 + C = 2CO. 



The conditions necessary for the formation of carbon monoxide are, 

 consequently, present in any stove or furnace where coal burns with 

 an insufficient supply of air. The carbon dioxide formed in the lower 

 parts of the furnace is decomposed by the coal above. The blue 

 flames frequently playing over a coal fire are burning carbon mon- 

 oxide. This gas is formed also by the decomposition of oxalic acid 

 (and many other organic substances) by sulphuric acid : 

 H 2 C 2 O 4 + H 2 SO 4 = H 2 SO,.H 2 O + CO 2 + CO. 



Oxalic Sulphuric 



acid. acid. 



Carbon monoxide is now manufactured on a large scale by causing the de- 

 composition of steam by coal heated to red heat. The decomposition takes 



place thus: 



H 2 + C == 2H + CO. 



