PHOSPHORUS AND ITS COMPOUNDS 283 



CHIEF POINTS OF CHAPTER XX. 



Phosphorus, like sulphur, exists in different allotropic forms. 



Ordinary phosphorus is a waxy, translucent solid of a pale yellow 

 colour. It very readily inflames and should not, in consequence, be 

 handled, and when not in use should be kept immersed in water. It 

 glows in the dark, evolving white fumes. It is poisonous. It melts 

 at about 43 C. It is soluble in carbon disulphide. 



Red phosphorus is a dark brownish-red powder, insoluble in 

 carbon disulphide, and only ignites when strongly heated. It is 

 non -poisonous. It need not be kept in water. 



Phosphorus and Oxygen. When either ordinary or red phosphorus 

 is burnt in air or oxygen a compound called phosphorus pentoxide, 

 P 2 , is formed. It is a white amorphous powder which dissolves 

 very readily in water, to form an acid solution known as phosphoric 

 add, H S P0 4 . [P,O 5 + 3H,0 = 2H 3 PO 4 .] Phosphoric acid may be 

 obtained directly from phosphorus by oxidising with nitric acid. 



Phosphoric acid forms salts called phosphates. Since phosphoric 

 acid contains three atoms of displaceable hydrogen it is possible to 

 obtain three different sodium salts of phosphoric acid ; those salts 

 which contain atoms of hydrogen as well as sodium are known as 

 ct </'(/ salts, while those in which all the hydrogen atoms have been 

 displaced are called normal salts. 



Phosphorous Oxide, P 4 6 , is formed to a small extent when 

 phosphorus burns in air, the quantity increasing if the supply of air 

 is insufficient. It is a white solid with a garlic-like odour. With 

 water it forms phosphorous acid, H 3 P0 3 . 



Compounds of Phosphorus and Hydrogen. The most important of 

 these is phosphine or phosphoretted hydrogen, PH 3 . It is evolved 

 when phosphorus is heated with a strong solution of caustic soda. 

 It has an offensive smell of rotten fish. When prepared in the 

 manner descri bed in the chapter it is spontaneously inflammable, but 

 this is due to its admixture with another compound of phosphorus 

 and hydrogen, P 2 H 4 , which can be got rid of either by passing the 

 gas through a cooled receiver or by allowing it to stand over 

 water. 



Manufacture of Phosphorus. Phosphorus is usually prepared from 

 the residue obtained by burning bones. This residue is a phosphate 

 of calcium, Ca 3 (PO 4 ). 2 . By treating this compound with sulphuric 

 acid it is converted into phosphoric acid. After concentration the 

 phosphoric acid is heated with powdered coke in cast iron retorts, 

 when phosphorus distils over and is cooled by water. 



QUESTIONS ON CHAPTER XX. 



1. In what respects do red and yellow phosphorus differ, and in 

 what respects do they resemble one another ? What reasons have 

 we for believing they are chemically identical ? 



2. What is formed when phosphorus burns ? Give an accfount of 

 the appearance and properties of the product. 



