PREPARING OXYGEN 



21 



FIG. 11. 

 A Home-Made Ring Stand. 



Continue to push the scum to one side, so that the bright 

 surface of the tin is kept in contact with the air. If the scum 

 formed contains the tin and some- 

 thing taken from the air, do you 

 think the dish, its contents, and the 

 wire, taken together, will weigh more, 

 or less, than they did before the 

 heating? Keep up the heating for, 

 say, 15 minutes, then let the ap- 

 paratus cool, and weigh it again. 

 What is the result? How much 

 matter was taken from the air? 



b. If you have no balances, carry out the heating without the 

 weighing. You can do the experiment at home, if necessary, 

 using a box cover for the iron dish, and the stove for the source 

 of heat. 



EXERCISE 17 

 PREPARING OXYGEN 



Apparatus and Materials. Wide-mouth bottle, stopper (two-hole), 

 dropping funnel (or ordinary form), glass rod, delivery tube, pan of 

 water; potassium permanganate or manganese dioxide, hydrogen 

 peroxide, splinter, candle, limewater. 



a. Prepare oxygen, if possible, in the ap- 

 paratus shown in Fig. 44, 51, of the text. If 

 you have no dropping funnel, you can use an 

 ordinary funnel with a glass rod to control the 

 size of the opening into the stem (Fig. 12). A 

 short piece of rubber tubing placed over the end 

 of the rod will make a tighter joint than the 

 glass alone. 



b. Simpler Form. You can make the oxygen 

 FIG. 12. more easily if you put the potassium permangan- 



