96 NON-METALS AND THEIE COMBINATIONS. 



vapors is diminished, or even prevented, by vapors of essential 

 oils (oil of turpentine, for instance), ether, olefiant gas, and a 

 few other substances. 



Oxides of phosphorus. Two oxides of phosphorus are known 

 in the separate state. They are phosphorus trioxide or phosphorous 

 oxide, P 2 3 , and phosphorus pentoxide or phosphoric oxide, P 2 5 . The 

 first is obtained by slow oxidation of phosphorus, the second 

 by burning phosphorus in dry air or oxygen. Both oxides are 

 white solids, which combine readily with water, forming the cor- 

 responding acids. 



Phosphorous acid, H 3 P0 3 = 82. This acid is obtained by dis- 

 solving phosphorous oxide in water : 



P 2 3 + 3H 2 = 2H 3 P0 3 . 



It is a colorless, acid liquid, which forms salts known as phos- 

 phites ; it is a strong deoxidizing agent, easily absorbing oxygen, 

 forming phosphoric acid. 



Tests for phosphorous acid. 



1. Added to mercuric chloride, a white precipitate of mer- 

 curous chloride is formed. 



2. Added to silver nitrate, a black precipitate of metallic 

 silver is produced. 



3. After being heated with nitric acid, it shows reactions of 

 phosphoric acid. 



Phosphoric acids. Phosphoric oxide is capable of combining 

 chemically with one, two, or three molecules of water, forming 

 thereby three different acids : 



P 2 5 4- H 2 O = H 2 P 2 6 = 2HPO 3 Metaphosphoric acid. 

 P 2 O 5 + 2H 2 O = H 4 P 2 7 Pyrophosphoric acid. 



P 2 5 + 3H 2 O = H 6 P 2 O 8 = 2H 3 P0 4 Orthophosphoric acid. 



These three acids show different reactions, act differently 

 upon the animal system, and form different salts. 



Metaphosphoric acid, HP0 3 = 80 (Glacial phosphoric acid). 

 This acid is always formed when phosphoric oxide is dissolved 

 in water ; gradually and more rapidly on heating with water it 



