26 LABORATORY EXERCISES 



a. Put a cold flask or small-mouth bottle (Fig. 52, 61, of 

 the text) mouth down in a pan of water, and grasp the flask in 

 both hands, so as to warm it. What happens? Why? 



Warm the flask more, either by heating it carefully, or by 

 pouring warm water over it. Results? Then let the flask cool, 

 and note what happens. 



b. Set an empty drinking glass, mouth down, in a saucer 

 or pan of hot water, as is often done when washed dishes are 

 drained. Explain all that happens, both at first and afterward, 

 when the glass and the water cool to the ordinary temperature. 



c. A cup, upside down, is usually placed in the center of a 

 meat pie. Find out why. 



EXERCISE 22 

 MELTING POINT AND FREEZING POINT; A FREEZING MIXTURE 



Apparatus and Materials. Beaker or measuring cup, thermometer, 

 crushed ice, salt, test tube. 



a. Into a beaker or a measuring cup put about 30 to 50 cu. 

 cm. of finely crushed ice, and stir it with a thermometer. Keep 

 the thermometer bulb entirely in the ice. Stir the ice until the 

 mercury stops shrinking; that is, until the thermometer ceases 

 to "fall." Then get the exact reading, and put it down. 



b. When you are sure that your reading of the melting point 

 is final, add to the ice a tablespoonful of salt, and again stir the 

 mixture with the thermometer. What is the lowest tempera- 

 ture reached by the thermometer? For what are freezing 

 mixtures used? 



c. Into the freezing mixture of b put a test tube containing 

 about 5 cu. cm. of water. Rinse off any salt that may be on 

 the thermometer, and with the . thermometer gently stir the 

 water in the test tube. Keep the bulb of the thermometer 



