24 LABORATORY EXERCISES 



what fractional part of the air has disappeared? Is it J^, or 

 ]/3, or what? What gas was taken out of the air that was in 

 the test tube? What gas remains? 



c. Put the beaker and test tube into a pail of water, so that 

 you can remove the test tube from the beaker without getting 

 the mouth of the test tube above the liquid. Then close the 

 mouth of the test tube with your thumb, remove the tube 

 from the water, and turn the tube right side up. Put a burning 

 match into the gas. Does the gas burn? Does the match con- 

 tinue to burn? How can you distinguish between nitrogen, air, 

 oxygen, and illuminating gas? 



d. If you wish to get a more exact result for the amount of 

 oxygen in air, measure in a graduated cylinder the amount of 

 water that entered the test tube. Then, leaving the tin in the 

 tube, fill the tube entirely with water, and get the volume of 

 the water. This equals the volume of the air originally in the 

 tube. From the results find the per cent of oxygen in air; that 

 is, the number of parts of oxygen in every 100 parts of air. 



e. Iron filings and 10% sodium hydroxide solution may be 

 used instead of tin and potassium hydroxide, but the reaction is 

 somewhat more slow. Wet the inside of the test tube with 

 10 cu. cm. of 10% sodium hydroxide solution, and pour the 

 solution into a beaker. Put into the tube about 1 cu. cm. of 

 iron filings, spreading them out in the closed end of the tube. 

 Then set the tube, mouth down, into the beaker, and let the 

 apparatus stand for one or two days. 



EXERCISE 20 

 EXPANSION AND CONTRACTION OF WATER 



Apparatus and Materials. Flask with one-hole stopper and long 

 glass tube, gummed paper (or thread), ring stand, flame, pail of hot 

 water, long-necked bottle. 



