METHODS AND MANIPULATION 7 similarly a sharp boiling-point which will remain constant until the whole of the liquid has boiled off, leaving no residue. Unlike the melting-point, however, this boiling-point, whilst remaining sharp, may vary in value over a range of several degrees, owing to fluctuations in the barometric pressure. The boiling-point of an impure liquid will depend largely on the physical nature of the impurities. If all the impurities are non-volatile, the liquid will have a sharp boiling-point, and the solid impurities will remain behind when the liquid has evaporated. If the impurities are themselves volatile, then the boiling-point of the liquid may ( a ) remain constant (see below), or ( b ) rise steadily as the liquid boils, or ( c ) rise in a series of definite steps, according to the nature and quantity of the impurities present. Although a pure liquid has a sharp boiling-point, the converse is not necessarily true: a sharp boiling-point does not always indicate a pure liquid, but may be caused by a constant-boiling
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