1G-19] PRECIPITATION AND FILTRATION 9 



white precipitate of calcium carbonate will be formed. 

 Reserve the test-tube and contents for use in paragraph 1 7. 



16. Filtration. — Very often it is necessary to separate 

 a solid from a liquid in which it is suspended. This 

 operation is performed by filtration or decantation (para- 

 graph 19). In the former case the liquid and solid are 

 thrown upon porous paper, the liquid passes through the 

 paper, the solid being retained. Unsized paper, known as 

 filter-paper, is used for this purpose. It is usually supplied 

 cut into circular pieces. 



17. Fit a filter-paper 

 into a funnel (fig. 13) by 

 folding it across twice at 

 right angles (fig. 12), so as 

 to form a conical bag as 

 shown at 3. Press this 

 bag into the dry funnel, 

 and then moisten the paper 

 with distilled water. The 

 precipitate and liquid from 

 paragraph 15 may then be 

 poured into the funnel, 

 when the liquid passing iaa ^™" FlG n 

 through will be quite clear, 

 the solid remaining on the filter-paper. 



18. Washing Precipitates.— Precipitates may be 

 washed free from adhering solutions either by decantation or 

 on the filter-paper, or by a combination of the two processes. 



19. Washing by Decantation.— Prepare a little 

 more precipitate, as described in paragraph 15 j allow it to 

 stand until the precipitate subsides, then pour off the clear 

 supernatant liquid. Add a little distilled water, shake up, 

 allow the solid to again subside, and pour off the clear 

 liquid. Repeat this operation four or five times, when all 

 the soluble matter will be washed out 



