Boiling Point, the temperature at which a liquid is converted into vapor with ebullition. It varies with the nature of the liquid and with the degree of pressure upon it, but it is ordinarily understood to mean that temperature at which the boiling occurs when the surface of the liquid is exposed to an atmospheric pressure equal to maintaining a column of mercury 29.922 inches in height. It is, consequently, the point at which the tension of the vapor is equal to the pressure upon the liquid. During the boiling of a liquid in the open air, therefore, the temperature remains constant, even when the amount of heat supplied to the liquid is increased. The additional heat, instead of being retained, is expended in converting an increased quantity of the liquid into vapor. If pure water is boiled in an open metallic vessel when the barometer stands at 29.922 inches, it will be observed that the ebullition takes place and continues, for a long time at least, at 212° F. If we substitute alcohol for water, ebullition will commence at 173°; and if sulphuric ether is used, its boiling point will be found at 95°, a temperature below that of the human body.
There are several bodies which at ordinary temperatures are gases, but which by the abstraction of heat or subjection to pressure, or both, may be reduced to liquids, whose boiling points are therefore below the ordinary temperature of the atmosphere. The following table gives the boiling points of several of both these classes of bodies, and also the atmospheric pressure at which the observations were made, and the authority:
Boiling point, F.
Height of barometer.
Chloride of ethyl......
It will be observed that the first four of the bodies in the above table are gases at temperatures below the freezing point of water, one of them passing into the liquid state only at 126.22° F. below zero. - The following method for ascertaining the boiling points of liquids is recommended by Prof. Kopp, and is particularly applicable to cases where the liquid is expensive, or where only a small quantity can be obtained. A small test tube is fitted with a cork through which are bored two small holes. Through one of these a delicate thermometer is passed, and through the other a bent glass tube, open at both ends. A few scraps of recently heated platinum foil are placed in the test tube, and then the liquid, only a small quantity of which is required, is poured in. The scraps of platinum foil are for the purpose of furnishing starting points for the formation of the steam bubbles. The bulb of the thermometer is usually placed in the vapor immediately above the liquid. A spirit lamp will quickly cause ebullition, the stearn passing off through the open glass into a cooled receiver. (See fig. i.) - Water has been the subject of very careful experiments with regard to its boiling point.
In consequence of the diminution of the weight of the atmosphere as we ascend to high mountain altitudes, the boiling point of water becomes so low that food cannot be cooked in it. Darwin, who as-cended one of the mountains of Patagonia, was unable to cook potatoes by boiling, and various travellers have ascended heights where it was impossible to boil eggs. At the city of Mexico, which is 7,000 ft. above the level of the sea, water boils at 200° F.; at Quito, which has an elevation of 9,000 ft., it boils at 194°; and at a height of 18,000 ft. in the Himalaya mountains Dr. Hooker found the boiling point to be 180°. In mines below the level of the sea water will not boil till it is raised to a temperature above 212° F. When the barometer marks 28.2 inches ebullition commences at 209 , so that the time required to cook food by boiling, even in the same locality, will often vary considerably. The boiling point of water under various degrees of atmospheric pressure, and consequently at various mountain altitudes, may be readily obtained by placing a vessel of warm water containing a thermometer under the receiver of an air pump, through the top of which has been introduced a barometer. (See fig. 2.) If the water in the vessel has been raised to 212 just before being placed under the receiver, it will require but a stroke of the piston of the air pump to produce ebullition.
By continuing the exhaustion the boiling may be rendered very violent, and then the mercury in the thermometer will be observed to fall very rapidly. The conversion of the water into vapor causes the conversion of sensible into latent heat, a term which is still retained, although modern theory regards it as being converted into mechanical force. When the water boils at 186° F., the column of mercury in the barometer will stand at about 17.5 indies, or about the same as at the summit of Mont Blanc, at an altitude of about 15,700 ft. above the level of the sea. By using a large pump and a small receiver, which may be quickly exhausted, and also a small quantity of water, placed in a test tube or a vessel of that form, and some strong sulphuric acid or chloride of calcium, for absorbing moisture, ebullition may be produced at a temperature as low as 45° F., or even lower. If it were possible to produce a perfect vacuum, it could be continued till the freezing point is reached; but the circumstances of the case prevent it. An apparatus like that represented in fig. 3 will serve to exhibit the effect of increased pressure on the boiling point.
A small iron boiler, a, having a thermometer, b, tightly adjusted, with the bulb passing to the interior, and furnished with a stopcock, c, receives at its mouth, d, a strong glass tube open at both ends, and sufficiently long to contain a column of mercury equal to the pressure it may be desired to produce. To the mouth a screw, through which the tube passes to near the bottom, is securely fitted. To make the experiment some mercury is poured into the boiler, and then it is about half filled with water, the bulb of the thermometer being left a little above the level. If now heat be applied while the stopcock is left open, the water will commence and continue to boil at 212° F.; but when the stopcock is closed the increased pressure produced by the confined steam will prevent ebullition unless the temperature is raised. When the mercury has been forced up the tube to a height of 30 inches, there will of course be a pressure of two atmospheres upon the surface of the water, the boiling point of which will be raised to 249°. If the heat be increased until the column attains a height of 90 inches, the pressure will be equal to four atmospheres, and the boiling point will be raised to 291°. Regnault, in his celebrated experiments, used a stronger and more complex apparatus than this, and found that at a pressure of 20 atmospheres the boiling point of water was 415.4° F. From the foregoing considerations it will be seen that a perpendicular column of water will have various boiling points at different depths.
Thus, if a column of water is 34 ft, in height, the particles at the bottom will sustain a pressure of two atmospheres, and it will require the application of 249° of heat to produce ebullition at that point, and of 234° at half the depth. When steam bubbles, having a temperature much above 212°, ascend through a column of liquid in a tall cylinder, they impart their excess of heat to it, and violent bursts of steam and boiling water are thrown from the mouth of the vessel. If a basin is placed about the orifice to catch the falling liquid, which in the presence of the expanding vapor has parted with much of its heat, and convey it back again to the cylinder, a period of comparative quiet will follow. During this time the temperature of the column will increase, and bubbles of steam will rise higher and higher, until at last, when they have attained sufficient force, the violent expulsion of steam and water will be repeated. The geysers in Iceland, and the great American geysers at the head waters of the Missouri river, are examples in nature of the boiling of water in vertical tubes. - There are some circumstances attending the boiling of water besides external pressure which must be taken into consideration in making experiments, or correct results will not le reached.
If water is boiled in a well cleaned glass flask which is perfectly smooth inside, it will, when the barometer stands at 29.922 inches, reach a temperature of 214°. If the flask had been rinsed with a solution of potash, the boiling might not have occurred below 215° or 216°. The reason assigned for these phenomena is that the perfect cleaning of the glass in one case, and the presence of a small quantity of potash in the other, increases the cohesion of the water and glass to such a degree as to demand an increase of heat to effect a separation between them. If water be boiled for a long time in a flask, and not in a vessel where the surface is freely exposed to the air, it will be observed, especially if the heat is moderately applied to the centre of the bottom, that the ebullition becomes more or less irregular or jerking. If the water is allowed to cease boiling for a few moments, and the heat is carefully applied, the temperature may be raised as high as 220° before any bubbles of steam will be formed, when the boiling will take place with a sudden leap, accompanied by a rapid decrease of temperature; then there will be another period of quietude, succeeded by another violent evolution of vapor.
These effects are heightened, if instead of using an open flask the water is boiled in a partial vacuum of its own vapor. This may be done by removing the lamp and corking the neck of the flask after the air has been as far as possible expelled. If we now turn cold water over the flask, the vapor within will be partially condensed, and the boiling will recommence and will continue even if the flask be plunged into cold water, until its temperature is reduced much below blood heat, and indeed as long as the tension of the vapor above the water can be kept below the tension of the vapor which the water is capable of yielding. Near the conclusion the ebullition becomes very irregular and jerking; and if the flask is placed in a retort stand and gently heated at the bottom, the bursts of vapor will be more explosive than during the cooling process, and sometimes the flask will be thrown from the stand. The explanation which is generally received is this: Water in its natural state contains a considerable quantity of atmospheric air. Boiling expels a portion, but not all of it, unless it has continued a long time.
While this expulsion of air is taking place, if only in exceedingly small quantities, little bubbles of it are formed into which the steam can enter and expand; but when the air is all expelled, the molecules of water will not separate from each other as readily as they passed into the air chambers. It seems as if there needed to be an opening or a point of diminished pressure somewhere in order that the particles of water at 212° F. may expand into vapor. Dufour has very carefully studied this subject. In experimenting with water he used a mixture of oil of cloves and linseed oil, which had been previously heated to 390° F. and allowed to cool. The water, heated to 170°, was carefully dropped in so as not to disturb the film of oil which coated the bottom of the vessel, and the temperature was gradually raised. The boiling point would invariably be passed and a heat of 230° or 236° reached before any manifestation of ebuDition could take place. Then an explosion would occur and the remainder of the globule of water would be violently driven to one side. He succeeded in raising some small glohules to 347° F., a temperature which would cause water with an exposed surface to boil under a pressure of more than eight atmospheres.
The passage of sparks from a Leyden jar would produce violent explosions; so also would a weak galvanic current, but in a less degree. In the latter case Dufour attributed the effect to the production of bubbles of gas at the ends of the conducting wires. He also found that when the surface of water was covered with a thin film of oil its temperature could be raised considerably above the boiling point. The investigations of Prof. Donny of Ghent, who has succeeded in raising water far above its boiling point when not enclosed in oil or other sub-stances, have added much to the stock of knowledge on the subject. Prof. Kopp and others have extended researches to various other liquids, and have found that many of them also possess the property of being raised under certain circumstances several degrees above their boiling points. Thus, methylic alcohol, whose boiling point is 141.8° F., may be raised by changing the nature of the vessel to 152°. In estimating the boiling point of a liquid Dufour very sensibly suggests that we should take the lowest temperature at which a liquid can be made to boil under the proper conditions. That an examination of this subject in relation to the cause of steam-boiler explosions would lead to important improvements is most probable.
That the temperature of the water in the boiler of a steam engine may be raised considerably above the boiling point is very possible, as for instance when the engine has been standing quiet for some time, and the water has been deprived of most of its air. Under such circumstances a disturbance of rest would cause an explosive burst of vapor, proportional to the temperature the water had attained. The presence of various salts in solution affects the boiling to a very great degree, but there has not been found much accordance between the solubility of the salts and the extent of their influence.
Table Of Boiling Points Of Saturated Solutions.
It has been a subject of controversy whether the vapors which issue from boiling aqueous solutions are of a higher temperature than the boiling point of pure water. According to the recent experiments of Prof. Magnus of Berlin the bubbles have at the moment of issuing a temperature equal to that of the highest stratum of the liquid; but it is almost instantaneously reduced by the absorption of heat occasioned by the expansion of the vapor. - All the observations that have been made fail to establish any relation between the boiling points of liquids and their specific gravities. Thus, bromine, with a specific gravity of 3.1862, boils at 145.4° F., while bromide of silicon, with a specific gravity of 2.8128, has a boiling point of 308°; and formic ether, having a specific gravity of .9357, boils at 127.7°, while fusel oil, with a specific gravity of only .8271, does not boil below a temperature of 269.8. The chemical constitution of many liquids, however, according to the investigations of Prof. Kopp, bears a very striking relation to their respective boiling points. He found that analogous compounds, having the same differences of composition, often have the same differences in their boiling points.
Thus, in the series of homologous acids which differ in composition by one molecule of CH2, and the alcohols from which they are derived by oxidation, he found that there was a difference of very nearly 34.2° F. in the boiling points. In the following table, which exhibits some of Kopp's results, it will moreover be observed that the difference in boiling points between each alcohol and its derived acid is very nearly 72° F.
Calculated boiling point, F.
Observed boiling point, F.
Kane, 140°; Kopp, 149°; Pierre, 150.8°.
Dumas, 163.8°; Gay-Lussac, Kopp, 172.4o.
Pierre, Kopp, 269.6°; Reickher, 275°.
Calculated boiling point, F.
Observed boiling point, F.
Liebig, 210.2°; Kopp, 221°.
Kopp, 242-6°; Sebille, Auger, 246.2°.
Propionic acid ...........................
Dumas. Leblanc, 234°; Kopp, 287.6°.
Kopp, Delffs. 312.8°; Pierre, 325.4°.
Dumas, Delffs, 347°; Kopp, 348.8°.
It was found that in the series of hydrocarbons homologous with benzole, C6H5, a difference of CH2 in chemical composition is accompanied with an average difference of about 43° F. in the boiling point; and in the series of alcohol radicles homologous with ethyl the difference in the corresponding boiling points was observed to be about the same.