OH, xx PHOSPHORUS AND ITS COMPOUNDS 279 



In this experiment the evaporation of the solvent leaves the 

 phosphorus deposited on the filter paper in so finely divided a 

 state that it rapidly oxidises, charring the paper, even setting it 

 alight. By careful evaporation of the solvent crystals may be 

 obtained. 



Red Phosphorus. This allotropic form of phosphorus is a 

 dark, brownish-red powder. Examine it carefully and note well 

 the differences from the ordinary variety, substituting red 

 phosphorus, and try again the various experiments performed. 

 It will be found to be insoluble in carbon disulphide, and to 

 only ignite when strongly heated (240 3 C.). It is not luminous, 

 neither does it oxidise when exposed to moist air. It is 

 therefore not necessary to keep red phosphorus under water. 

 Red phosphorus, or amorphous phosphorus, as it is also called, 

 is, further, non-poisonous. 



Phosphorus and Oxygen. We have stated before that when 

 phosphorus burns it forms a compound called phosphorus pent- 

 oxide. This is the product of combustion when either red or 

 yellow phosphorus is used, and this fact may serve to prove that 

 the two varieties are chemically identical. The composition of 

 the oxide is represented by the formula P 2 O 5 , the prefix pent 

 (five) indicating the number of atoms of oxygen in the molecule 

 of the compound. The properties of this oxide have been 

 already partially studied (p. 222). It is a white amorphous 

 powder which dissolves very readily in water, forming an acid 

 solution. 



If left exposed to the air it absorbs moisture, being, in fact, 

 one of the most powerful of drying agents, on account of which 

 it is frequently used as a dehydrator (i.e., for the purpose of 

 removing moisture from gases or liquids). For the same reason 

 it is of service in promoting many chemical reactions, the 

 essential part of which is the removal of the elements of water. 



Phosphoric Acid and Phosphates. The acid solution which 

 results on the solution of the oxide, P 2 O 5 , in water is known as 

 phosphoric acid, 1 and its composition is represented by the 

 formula H 3 PO 4 , so that we may write the equation for its pro- 

 duction thus : 



We may also obtain the acid directly from phosphorus itself by 



i This acid results if the water be hot : if cold water be used, another acid, which 

 gradually changes to this, results. 



