164 NON-METALS AND THEIR COMBINATIONS. 



In regard to the laws of osmotic pressure, deviations from them are observed 

 in the case of aqueous solutions of the same substances for which the freezing- 

 point and boiling-point laws do not hold, namely, acids, bases, and salts. 

 These always show greater osmotic pressures than those calculated, and than 

 that shown by cane-sugar, which is a type of normally acting substance. The 

 deviations are explained by the same assumption that is made to explain devi- 

 ations from the freezing-point law (see above), namely, decomposition of mole- 

 cules into a greater number of particles, that is, ions. 



13. NITROGEN. 

 Niii = 14 (13.93). 



Occurrence in nature. By far the larger quantity of nitrogen is 

 found in the atmosphere in a free state. Compounds containing 

 nitrogen are chiefly the nitrates, ammonia, and many organic sub- 

 stances. 



Preparation. Nitrogen is obtained usually from atmospheric air 

 by the removal of its oxygen. This may be accomplished by burn- 

 ing a piece of phosphorus in a confined portion of air, when phos- 

 phoric oxide, a white solid substance, is formed, while nitrogen is left 

 in an almost pure state. 



Other methods for obtaining nitrogen are by heating a mixture of 

 potassium nitrite and ammonium chloride dissolved in water : 



KNO 2 -f NH 4 C1 = KC1 -f 2H 2 O + 2N; 

 or by heating ammonium nitrite in a glass retort : 

 NH 4 NO 2 = 2H 2 + 2N. 



Experiment 5. Use an apparatus as shown in Fig. 37, page 140. Place in the 

 flask about 10 grammes of potassium nitrite and nearly the same amount of 

 ammonium chloride; add enough water to dissolve the salts, and apply heat, 

 which is to be carefully regulated from the time the decomposition begins, as 



QUESTIONS. Give a definition of solution and its general characteristics. 

 What are immiscible solvents and how are they employed? Define dilute, 

 concentrated, and saturated solutions. What is meant by the solubility of a 

 substance, and by heat of solution ? What is Henry's law regarding the solu- 

 tion of gases in liquids? What is the relation between the freezing-points of 

 solutions and the weights of dissolved substances? What use is made of the 

 cryoscopic method ? What is osmotic pressure? What is a semi-permeable 

 membrane? How do the laws of osmotic pressure compare with the gas laws? 

 What are isotonic, hypertonic, and hypotonic solutions ? What is the expla- 

 nation of the abnormal behavior shown by solutions of acids, bases, and salts 

 in their freezing-points, boiling-points, and osmotic pressures? 



