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The effect of passing bubbles through the ether is simply to increase enormously the evaporating surface, for the ether molecules which could before escape only at the upper surface can now escape into the air bubbles as well.
221. Factors affecting evaporation. The above results may be summarized as follows: The rate of evaporation depends (1) on the nature of the evaporating liquid; (2) on the temperature of the evaporating liquid; (3) on the degree of saturation of the space into which the evaporation takes place; (4) on the density of the air or other gas above the evaporating surface; (5) on the rapidity of the circulation of the air above the evaporating surface; (6) on the extent of the exposed surface of the liquid.
QUESTIONS AND PROBLEMS
1. Why do spectacle lenses become coated with mist on entering a warm house on a cold winter day?
2. Does dew "fall"?
3. Why are icebergs frequently surrounded with fog?
4. Dew will not usually collect on a pitcher of ice water standing in a warm room on a cold winter day. Explain.
5. The dew point in a room was found to be 8° C. What was the relative humidity if the temperature of the air was 10° C.? 20° C.? 30° C.? (Consult table, p. 171.)
6. What weight of water is contained in a room 5 × 5 × 3 m. if the relative humidity is 60% and the temperature 20° C.? (See table, p. 171.)
7. If a glass beaker and a porous earthenware vessel are filled with equal amounts of water at the same temperature, in the course of a few minutes a noticeable difference of temperature will exist between the two vessels. Which will be the cooler, and why? Will the difference in temperature between the two vessels be greater in a dry or in a moist atmosphere?
8. Why will an open, narrow-necked bottle containing ether not show as low a temperature as an open shallow dish containing the same amount of ether?
9. Why is the heat so oppressive on a very damp day in summer? 10. A morning fog generally disappears before noon. Explain the reason for its disappearance.
11. What becomes of the cloud which you see about a blowing locomotive whistle? Is it steam?
12. Explain why it is necessary in winter to add moisture to the air of our homes to maintain proper relative humidity, but not necessary in the summer.
13. What factors affecting evaporation are illustrated by the following: (1) a wet handkerchief dries faster if spread out, (2) clothes dry best on a windy day, (3) clothes do not dry rapidly on a cold day, (4) clothes dry slowly on humid days? Explain each fact.
222. Heat of vaporization defined. The experiments performed in Chapter IV, Molecular Motions, led us to the conclusion that, at the free surface of any liquid, molecules frequently acquire velocities sufficiently high to enable them to lift themselves beyond the range of attraction of the molecules of the liquid and to pass off as free gaseous molecules into the space above. They taught us, further, that since it is only such molecules as have unusually high velocities which are able thus to escape, the average kinetic energy of the molecules left behind is continually diminished by this loss from the liquid of the most rapidly moving molecules, and consequently the temperature of an evaporating liquid constantly falls until the rate at which it is losing heat is equal to the rate at which it receives heat from outside sources. Evaporation, therefore, always takes place at the expense of the heat energy of the liquid. The number of calories of heat which disappear in the formation of one gram of vapor is called the heat of vaporization of the liquid.
223. Heat due to condensation. When molecules pass off from the surface of a liquid, they rise against the downward
* It is recommended that this subject be accompanied by a laboratory determination of the boiling point of alcohol by the direct method and by the vapor-pressure method, and that it be followed by an experiment upon the fixed points of a thermometer and the change of boiling point with pressure. See, for example, Experiments 23 and 24 of the authors' Manual.
forces exerted upon them by the liquid, and in so doing exchange a part of their kinetic energy for the potential energy of separated molecules in precisely the same way in which a ball thrown upward from the earth exchanges its kinetic energy in rising for the potential energy which is represented by the separation of the ball from the earth. Similarly, just as when the ball falls back it regains in the descent all of the kinetic energy lost in the ascent, so when the molecules of the vapor reënter the liquid they must regain all of the kinetic energy which they lost when they passed out of the liquid. We may expect, therefore, that every gram of steam which condenses will generate in this process the same number of calories as was required to vaporize it. This is the principle of the steam heating of buildings, by which the heat energy that disappears in converting the water in the boilers into steam is generated again when the steam condenses to water within the radiators.
224. Measurement of heat of vaporization. To find accurately the number of calories expended in the vaporization, or released in the condensation, of a gram of water at 100° C., we pass steam rapidly for two or three minutes from an arrangement
like that shown in Fig. 177 into a vessel containing, say, 500 g. of water. We observe the initial and final temperatures and the initial and final weights of the water. If, for example, the gain in weight of the water is 16.5 g., we know that 16.5 g. of steam have been condensed. If the rise in temperature of the water is from 10° C. to -30°C., we know that 500 × (30-10)=10,000 calories of heat have entered the water. If
tion of water
x represents the number of calories given FIG. 177. Heat of vaporizaup by 1 g. of steam in condensing, then the total heat imparted to the water by the con
densation of the steam is 16.5 x calories. This condensed steam is at first water at 100° C., which is then cooled to 30° C. In this cooling
process it gives up 16.5 × (100–30) = 1155 calories. Therefore, equat‐ ing the heat gained by the water to the heat lost by the steam, we have
10,000 = 16.5 x + 1155, or x = 536.
This is the method usually employed for finding the heat of vaporization. The now accepted value of this constant is 536. 225. Boiling temperature defined. If a liquid is heated by means of a flame, it will be found that there is a certain temperature above which it cannot be raised, no matter how rapidly the heat is applied. This is the temperature which exists when bubbles of vapor form at the bottom of the vessel and rise to the surface, growing larger as they rise. This temperature is commonly called the boiling temperature.
But a second and more exact definition of the boiling point may be given. It is clear that a bubble of vapor can exist within the liquid only when the pressure exerted by the vapor within the bubble is at least equal to the atmospheric pressure pushing down on the surface of the liquid; for if the pressure within the bubble were less than the outside pressure, the bubble would immediately collapse. Therefore the boiling point is the temperature at which the pressure of the saturated vapor first becomes equal to the pressure existing outside.
226. Variation of the boiling point with pressure. Since the pressure of a saturated vapor varies rapidly with the temperature, and since the boiling point has been defined as the temperature at which the pressure of the saturated vapor is equal to the outside pressure, it follows that the boiling point must vary as the outside pressure varies.
Thus let a round-bottomed flask be half filled with water and boiled. After the boiling has continued for a few minutes, so that the steam has driven out most of the air from the flask, let a rubber stopper be inserted and the flask removed from the flame and inverted as shown in Fig. 178. The temperature will fall rapidly below the boiling point; but if cold water is poured over the flask, the water will again begin to boil vigorously, for the cold water, by condensing the steam, lowers the
pressure within the flask, and thus enables the water to boil at a temperature lower than 100° C. The boiling will cease, however, as soon as enough vapor is formed to restore the pressure.
The operation may be repeated many times without reheating.
At the city of Quito, Ecuador, water boils at 90° C.; on the top of Mt. Blanc it boils at 84° C.; and on Pikes Peak, at 89° C. On the other hand, in the boiler of a locomotive on which the gauge records a pressure of 250 pounds, as is frequently the case, the boiling point of the water is 208° C. (406° F.).
FIG. 178. Lowering the boiling point by diminishing the pressure
Closed boilers provided with safety valves (see C, Fig. 179) and known as digesters are used for more rapid cooking in mountainous regions. Indeed, a temperature only a few degrees above 100° C. causes starch grains to burst open much more rapidly than does a temperature of 100° C. Large digesters are used in extracting gelatin from bones and in reclaiming valuable fatty substances at garbage plants. In the cold-pack method of preserving fruits and vegetables the final sterilizing is done by placing the jars or cans in closed boilers known as steampressure canners.*
FIG. 179. A closed boiler for family use
227. Evaporation and boiling. The only essential difference between evaporation and boiling is that the former consists in the passage of molecules into the vaporous condition from the free surface only, while the latter consists in the passage of the molecules into the vaporous condition both at the free surface and at
* Farmers' Bulletin No. 839, on steam-pressure canning, may be obtained from the United States Department of Agriculture, Washington, D. C.