LESSONS IN CHEMISTRY 



11 



2. A STUDY OF AIR 



Preparation of Oxygen. 1 In the preceding study of match- 

 burning, frequent reference has been made to oxygen as one 

 of the elements found in the air. To study the peculiar 

 properties of this gas, we must obtain it in a free state; 

 we can readily get it in this form from compounds that con- 

 tain a large amount of oxygen. One of these is the ordi- 

 nary chlorate of potassium. 



FIG. 1. Preparation of Oxygen. 



We first mix some of the white crystals of this chlorate 

 of potassium with about one third this amount of a black 

 powder called oxid of manganese. Then we put the two 

 into a test tube and close its opening with a stopper through 

 which is passed a bent glass delivery tube (see Fig. 1). The 

 other end of the latter dips beneath the water in a collect- 

 ing tray. When heat is applied to the test tube, the white 

 crystals and black powder are made to give up some of the 

 oxygen which they contain. The gas passes through the 

 delivery tube; and since it does not readily dissolve in 

 water, it comes off in bubbles and escapes into the air. 



Atmospheric Pressure. 2 Suppose now we fill a wide- 

 mouthed bottle with water, cover the top with a square of 

 glass (Fig. 1), and invert it in the tray of water, removing 



1 See "Laboratory Exercises," No. 2. 



2 See " Laboratory Exercises," No. 4. 



