EXAMINATION OF COMMON SUBSTANCES 



The suspended chalk can also be removed by pouring the milky liquid 

 on to a piece of porous paper (filter paper) supported in a funnel. Note 

 the method of folding the paper (Fig. 71). 



FIG. 71. How to use a round piece of blotting-paper for filtering. 



The method of pouring a liquid into a filter is shown in Fig. 72. (Never 

 attempt to pour a liquid into a funnel without using a rod as shown.) A 

 clear liquid passes through the filter paper, and it is evident that most 

 of the chalk has not dissolved, 

 but it is possible that some of it 

 may have. Pour a little of the 

 clear liquid into an evaporating 

 basin and heat over a Bunsen 

 burner on a wire gauze. The 

 water is driven off as steam, 

 and no powder remains, showing 

 that chalk is insoluble in water. 



A more rapid way of evapor- 

 ating a little of the solution is 

 to dip a clean stirring rod into 

 it and hold the wet rod above 

 the flame. If the rod remains 

 clean there is no solid dissolved 

 in the solution. 



iii. Structure of chalk. 

 Shake up a small quantity of 

 powdered chalk with water in a 

 test-tube. Pour off the water 

 containing suspended chalk, and 

 examine a portion of the sedi- 

 ment from the bottom of the 

 tube by placing a few particles 

 on a glass slip, covering it with 

 a suitable " cover slip " and 

 examining under a microscope. 



Note. The chalk you have 

 examined is not the substance 

 which is used for writing on the 

 blackboard. 



FIG. 72. How to pour a liquid into a paper 

 filter in a glass funnel. 



Chalk. A white, amorphous solid which is very easily powdered. 

 It does not dissolve in pure water, but it is slightly soluble in rain- 

 water owing fa the presence of carbon dioxide in the latter (p. 170), 



