Page:The New International Encyclopædia 1st ed. v. 03.djvu/274

BOILING-POINT. 240 BOILING-POINT. face of the water; the presence of air checks the speed of evaporation, though it cannot pre- vent it from going on until the vapor-pressure has finally attained its limit determined by the temperature. While the presence of air thus diminishes the rate of evaporation, an elevation of tempera- ture causes, on the contrary, a corresponding increase of that rate. As the temperature of the water rises, its vapor-tension continuously in- creases, and with it increases the rate of evapora- tion. Finally, when the temperature has risen to the point at which the vapor-tension is ■ equal to the external pressure of the air, evapora- tion becomes very rapid indeed, and bubbles of water form in brisk succession throughout the volume of tlie liquid, throwing it into more or less violent commotion ; the water is then said to be boiling. If the steam is now allowed to escape, boiling will go on until all the water has been evaporated; and if. while the liquid is boil- ing, the bulb of a mercury thermometer be placed near the surface of the water, the thermometer will register a constant temperature (provided the water is pure). This constant temperature, the lowest at which, under a given pressure, water will continue to boil, is called the boiling-point of water at that pressure. The so-called normal boiling-point of water is 100° Centigrade (212° F.). This is the tem- perature at which water boils under normal at- mospheric pressure; i.e. when the height of the mercury barometer is exactly 760 millimeters (30 inches). A change of pressure will at once result in a corresponding change of the boiling- temperature. In elevated positions, where the atmosphere is rare and the barometric pressure comparatively low, the boiling-point is lower than at the level of the sea. At the City of Mexico, 7000 feet above the sea. water boils at 93.3° C. (200° F.); at certain points in the Himalayas it boils at 82.2° 0. (180° F.). Boil- ing water is thus not always equally hot, and in elevated places many substances cannot be cooked by boiling. Once the boiling-temperatures of water corresponding to different heights have been ascertained, we can, conversely, determine the height of a mountain by observing the boil- ing-point of water at its summit. The above statements as to the boiling-point of water are true only of water which is chemi- cally pure and contains no admixture of any sort. If, instead, we take a solution, say, of ordinary salt, we will find: ( 1 ) that when heated under normal atmospheric pressure, the solution will begin to boil at some temperature higher than 100° C; and (2) that, whatever the tem- perature at which boiling will begin, if the steam is allowed to escape, the temperature of the boil- ing solution will continuously rise; in other words, the solution will not continue boiling at a constant temperature. When the 'boiling-point of a solution' is spoken of, it should be under- stood to mean the degree of heat at which boiling just commences. The only reason that a solution will not boil at constant temperature lies in the fact that, during the process of boiling, its com- position changes; for when a solution boils, water escapes in the form of steam, while the solid substance remains behind and its propor- tionate amount in the solution consequently in- creases. If, however, the steam is continually condensed by means of a suitable cooler, and thus made to return to the solution, the com- position of the latter will remain unchanged, and no matter how long boiling is kept up, the temperature will remain constant; this tem- perature is evidently the point at which other- wise, if the steam were allowed to escape, boiling would just commence. From what has been said of boiling solutions, it maj' be seen that the boiling-point of a liquid is the higher, the greater the amount of foreign substance contained in it. This is, however, not the only factor on which the temperature of a boiling solution depends. The molecular weight of the dissolved substance is another factor. If we were to take two equal quantities, say, of water, heat them to boiling and then add to them, respectively, equal weights of two different substances, we would find that the elevation of temperature is greater in the case of the sample to which we have added the substance of smaller molecular weight. Experi- ments carried out in this manner with many different substances permit us to induce the law, that the difference of the boiling-tempera- tures of a solution and of the pure solvent is inversely proportional to the molecular weight of the substance dissolved. It must, however, be borne in mind that the law is limited to solutions which do not conduct electricity; in solutions of electrolytes relations are not quite so simple. (See articles Solutions and Di.ssoci.- TION.) In the case of solutions of non-electro- lytes, such as many of the carbon compounds, the law holds with considerable precision; and it has proved especially useful in this, that it permits us to determine the unknown molecular weights of newly discovered substances. To as- certain, by this method, the molecular weight of a new .substance, all the chemist has to do is first to obsei-ve the rise of temperature produced in a boil- ing solvent, when a certain quantity of his substance is add- ed to it, and then to compare the rise of boiling - point to the rise produced by the same quantity of some other sub- stance, of known molecular weight. These important de- terminations are carried out with considerable preci- sion by the use of Beckmann's appa- ratus (see figure). Theapparatuscon- sists essentially of two parts : an inner tube, a, containing the solution, and an outer vessel, b, con- taining the pure sol- vent and separated from a near the bottom by a sheet of asbestos. The coolers c and d serve to condense the escaping va- pors. To determine the rise of boiling-point caused by a given substance, the operator introduces into a first a few cubic centimeters of the pure solvent, BECKMANN S APPABATUS.