88 CHEMISTRY FOR AGRICULTURAL STUDENTS 



XXII. NITRIC ACID AND THE NITRATES 



Nitrogen, as its name implies, is a constituent of nitre or 

 saltpetre. Insert a grain of this salt on a platinum wire into a 

 non-luminous flame. What base does the coloration of the 

 flame show that the salt is a compound of? Repeat the 

 experiment with "Chili saltpetre." In what respect do the 

 salts differ? Warm a little of each salt in test-tubes with 

 strong sulphuric acid. Do the fumes of the acid evolved 

 appear to be identical ? Are they acid to litmus ? Note that 

 the vapour causes the inflammation of a glowing splint, showing 

 that the acid is an oxygenated acid that readily loses oxygen ? 

 These experiments show that saltpetre and Chili saltpetre are, 

 respectively, the potassium and sodium salts of an oxy-acid 

 (so called to distinguish acids which are the hydrates of 

 acidic oxides from acids which are compounds of non-metallic 

 elements with hydrogen). Being obtained from nitre, the acid 

 is called nitric acid. It is the hydrate of nitric anhydride, 



NP5- 



N2O5-1-H2O-2HNO3. 



A solution containing one gram of potash (KgO) when exactly 

 neutralised with a solution of nitric acid, and evaporated to 

 dryness on a water bath, leaves a residue of nitrate of potassium 

 weighing 2.149 grams. What formula will represent the com- 

 position of the salt? Judging from analogy, sodium and 

 potassium having been found to resemble each other in all 

 their chemical properties, what will be the formula for nitrate of 

 sodium. 



Prepare some nitric acid. Place about 25 grams of 

 saltpetre in a retort (Fig. 14), and, by means of a thistle 

 funnel thrust down the neck, add to it about 15 c.c. of 

 strong sulphuric acid. Heat the mixture gently, using a rose 

 burner, and collect the distillate in a flask kept cool by water. 



Nitrogen is an element with apparently a weak attraction 



