398 PRACTICAL ORGANIC CHEMISTRY (a) Two liquids may be separated by fractional distillation provided that they do not form a constant-boiling mixture and that their boiling-points are sufficiently far apart. For instance, carbon tetrachloride (b.p. 77°) and toluene (b.p. 110°) can be separated in this way, but difficulty may be experienced in separating substances with much closer boiling-points, unless an efficient fractionating column is used. Two components can often be separated if one is volatile, and the other non-volatile, in steam, e.g., o-nitrophenol and />-nitrophenol (p. 170). Frequently one component of a mixture is readily sublimed and so can be obtained in a highly purified condition by this process. Solutions of solids in liquids can sometimes be separated by distilling off the liquid and leaving a residue of the solid, e.g., acetone and acetamide. (b) This depends entirely upon the nature of the particular compounds present, and no general rule can be given. The method of separation
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