HYDROGEN 35 



Does all dissolve? What evidence is there that you now have 

 a saturated solution? 



Heat the dish, and stir the alum and water until the solution 

 nearly boils. What happens? Set the solution aside to cool 

 slowly. Examine the crystals that are formed; do they have 

 any regular form? Ask whether you are to save the alum. 



What was the shape of the salt crystals formed in Exercise 

 8, d? 



EXERCISE 33 

 DOES THE TEMPERATURE CHANGE DURING SOLUTION? 



Apparatus and Materials. Thermometer, beaker or cup, water, 

 ammonium chloride (sal ammoniac), watch glass or glass plate. 



a. Get the temperature of 10 cu. cm. of water in a beaker or 

 cup. Hold the dish by the edge, so that your hand will not 

 warm the water. Then mix with the water a teaspoonful 

 (about 5 cu. cm.) of ammonium chloride (sal ammoniac), and 

 stir the mixture with the thermometer. Have the thermometer 

 bulb immersed, and hold the dish by the edge, as before. What 

 change of temperature is there? 



b. Pour a few drops of the sal ammoniac solution on a watch 

 glass or a glass plate, and let it evaporate slowly, without 

 applying heat. What form have the crystals? Draw a sketch 

 of some of them. 



Ask what you are to do with the sal ammoniac solution. 



EXERCISE 34 

 HYDROGEN 



Apparatus and Materials. Small, wide-mouth bottle or a test tube, 

 large bottle or fruit jar, a drinking glass, dilute sulphuric acid, granu- 

 lated zinc, copper sulphate solution, splinter, kettle or pail of cold 

 water, burner. 



