PREPARATION OF PURE COMPOUNDS 13 



EVAPORATION OF LIQUIDS. 



Only aqueous solutions can be evaporated over a flame, and 

 the evaporation should be completed over a water-bath to prevent 

 charring the substance as it becomes concentrated. Evaporation over 

 a flame must be carefully watched to prevent charring and also to avoid 

 spurting when the solution begins to concentrate. 



Most organic liquids which are used as solvents are readily inflam- 

 mable and must not be brought near a flame. 



When a considerable amount of solvent is present it is removed by 

 distillation and in cases where the solvent (ether, acetone, ligroin, etc.) 

 is very readily inflammable the distillation must be carried out on 

 a water-bath, heated by a flame specially protected by a gauze, or by 

 steam, or electrically. If the bath be heated by a burner, the end ol 

 the condenser should be placed as far away as possible and a sheet 

 of cardboard or asbestos interposed between the burner and receiver. 



Not only is evaporation by distillation absolutely necessary with 

 inflammable liquids, but also it is economical. The solvent is recovered 

 and after purification can be used again. 



When only small quantities of liquids up to 25 c.c. require evapora- 

 tion they are set aside, away from flames, and allowed to evaporate 

 spontaneously, or they may.be put upon a warm water-bath, with the 

 flame extinguished. 



Another common procedure is to evaporate small quantities by 

 placing them in a vacuum desiccator and exhausting. The evaporation 

 is greatly accelerated if the liquid be previously warmed on a water-bath 

 and whilst warm put into the desiccator. 



Evaporation in l/acuo. 



Evaporation in vacua is the most rapid method of concentrating solutions, 

 especially if the temperature can be kept at an elevated point. 



Very frequently evaporation of solutions must be carried out at a low tem- 

 perature (35-45) to prevent decomposition. This is carried out in the same 

 way as distillation in vacua, the distilling flask being kept in water at 35-45- 

 With large flasks of 2-3 litres capacity and a good vacuum of about 15 mm., 

 a litre of water can be distilled off in 2-2^ hours at 35-45. Instead of a distil- 

 ling flask an ordinary flask fitted with capillary and bent tube may be used and 

 a similar one as receiver. 



