BOILING-POINT. The temperature at which a liquid substance boils. Evaporation of water takes place. more or less rapidly, at all tem peratures (even at temperatures at which it rapidly freezes). no matter how great the pres sure of air or other gases on its surface. When some water is placed in a closed evap oration will go on until the escaping vapor has attained a certain definite pressure of its own. whose magnitude is dependent urn notldng but the temperature of the water; this pressure of water-vapor is called the ropor-tcasion of water at the given temperature. Now, though the magnitude of vapor-tension has nothing to do with the presence of absence of air in the vessel, the rate at which evaporation proceeds is greatly influenced by the pressure which air inclosed within the vessel may exert on the sur 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 the 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 790 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. (2011° F.) ; at certain points in the Himalayas it boils at S2.2° C. (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. lf, 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.