48 Preparation of Substances 



pure water is extremely slight. It is, how- 

 ever, readily decomposed by the salts which 

 appear in natural waters, the carbon dioxid of 

 hard water being particularly effective. The 

 amounts of arsenic acid set free by this reac- 

 tion are, generally, less than one per cent, 

 depending on the water used and not enuf to 

 burn foliage. 



QUESTIONS 

 (To be answerd in the notebook.) 



1. How many grams of anhydrous, 80 per cent 

 pure, arsenate of soda would be required to put with 

 20 grams of lead nitrate? 



2. Explain how lead nitrate can turn litmus red. 



3. Similarly, how sodium arsenate can turn litmus 

 blue. 



4. What is meant by the terms flocculent; de- 

 flocculated? 



5. Explain the action of soluble salts in flocculating 

 the precipitate. 



6. What salts are washt out of the mixture? How 

 did the litmus paper act in your preparation im- 

 mediately after mixing? 



7. How much arsenic acid (AS2O5) in an ordinary 

 lead arsenate paste? 



8. If lead costs more than arsenic which is cheaper 

 to use as a lead salt, the nitrate or the acetate? The 

 analyses of lead arsenates given \^'ill furnish the answer. 



9. Calculate the per cents of AsoOs and PbO in 

 PbHAs04 in the basic compound. This will be confus- 

 ing unless the student keeps in mind the fact that the 

 amount of two arsenics cannot be calculated where 

 only one exists, i.e., AS2O5 cannot be calculated from 



