CHAPTER XX 

 PRECIPITATION, FILTRATION, AND DISTILLATION 



119. Precipitates. When two liquids are poured to- 

 gether, one usually dissolves in the other, unless they happen 

 to be liquids that never mix, such as oil and water, but in 

 certain cases, two liquids, instead of forming a solution, make 

 new chemical combinations some of which are not soluble. 

 In such cases, the insoluble matter soon sinks to the bottom. 

 Matter thrown out of a solution in this way is called a precipi- 

 tate. Water and alcohol readily mix, but when alcohol has 

 camphor dissolved in it, the addition of water forces the 

 camphor out of the combination and causes it to appear in 

 the mixture as small white flakes. If the matter suspended 

 in a liquid is so finely divided as to settle very slowly, adding 

 other substances to it may cause the particles to come together 

 into larger groups and settle more rapidly. This process 

 is known as flocculation. Iron sulphate and alum are sub- 

 stances often used in clearing turbid water by flocculation. 

 Lime added to water in which clay is suspended will also cause 

 flocculation. The particles in clay soils may be flocculated 

 in the same way. 



120. Filters and Filtering. A precipitate may be separated 

 from the liquid containing it by passing it through sub- 

 stances containing many fine pores. Such substances are 

 known as filters. In the laboratory, filters are usually made 

 of a special paper, called filter paper, but other filters may be 

 made of charcoal, glass wool, stone, beds of sand, and the like. 

 Only precipitates, or suspended matter, can be separated from 



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