98 WATER 



to soak in the kerosene for some time. Pitch may be 

 removed from the hands with kerosene. The kerosene 

 may then be removed with soap and water. 



37. Crystals. 



We have seen that some solids can disappear in 

 solution. Do you think that they can come out of solu- 

 tion and appear again? The only way in which we can get 

 a solid out of solution is to drive off the water by means 

 of heat, called boiling, or we can let the water pass off 

 slowly and without bubbling. This last method is called 

 evaporation. Let us learn about this by some experi- 

 ments. 



Experiment 50. Crystallization. 



Apparatus: Burner, ring stand, wire gauze, two 

 beakers, three glasses, stirring rod, deep dish. 



Materials: Alum (powdered), copper sulphate, 

 common salt, string. 



a. To a little water add one half as much powdered 

 alum and stir for a few minutes. Does it all dissolve? 

 Now put it on the ring stand, resting it upon the wire 

 gauze, and heat it until it boils. Does heat aid solution? 

 Place the beaker containing the solution, in a dish of cold 

 water and stir it constantly until cold. What happens? 

 Heat the beaker again until all of the alum has dis- 

 solved and then place it again in the dish of cold water, 

 but do not stir it or touch it until it is cold. What is 

 the result? What you see are crystals. Which method 

 produces the larger crystals, rapid cooling or slow cool- 

 ing? Keep the crystals. 



