1 gram of water 1° C. is equal to 1 calorie, and the amount necessary to raise the temperature 100° C. is equal to 100 calories; hence the amount of heat necessary to convert 1 gram of water at the boiling point into steam at that same temperature is equal to approximately 525 calories. Very careful experiments show the exact heat of vaporization to be 536.1 calories. (See Laboratory Manual.)
22. General Truths. Statements similar to the above hold for other liquids and for solutions. If milk is placed upon a stove, the temperature rises steadily until the boiling point is reached; further heating produces, not a change in temperature, but a change of the water of the milk into steam. As soon as the milk, or any other liquid food, comes to a boil, the gas flame should be lowered until only an occasional bubble forms, because so long as any bubbles form the temperature is that of the boiling point, and further heat merely results in waste of fuel.
We find by experiment that every liquid has its own specific boiling point; for example, alcohol boils at 78° C. and brine at 103° C. Both specific heat and the heat of vaporization vary with the liquid used.
23. Condensation. If one holds a cold lid in the steam of boiling water, drops of water gather on the lid; the steam is cooled by contact with the cold lid and condenses into water. Bottles of water brought from a cold cellar into a warm room become covered with a mist of fine drops of water, because the moisture in the air, chilled by contact with the cold bottles, immediately condenses into drops of water. Glasses filled with ice water show a similar mist.
In Section 21, we saw that 536 calories are required to change 1 gram of water into steam; if, now, the steam in turn condenses into water, it is natural to expect a release of the heat used in transforming water into steam. Experiment shows not only that vapor gives out heat during condensation, but that the amount of heat thus set free is exactly equal to the amount absorbed during vaporization. (See Laboratory Manual.)
We learn that the heat of vaporization is the same whether it is considered as the heat absorbed by 1 gram of water in its change to steam, or as the heat given out by 1 gram of steam during its condensation into water.
24. Practical Application. We understand now the value of steam as a heating agent. Water is heated in a boiler in the cellar, and the steam passes through pipes which run to the various rooms; there the steam condenses into water in the radiators, each gram of steam setting free 536 calories of heat. When we consider the size of the radiators and the large number of grams of steam which they contain, and consider further that each gram in condensing sets free 536 calories, we understand the ease with which buildings are heated by steam.
Most of us have at times profited by the heat of condensation. In cold weather, when there is a roaring fire in the range, the water frequently becomes so hot that it "steams" out of open faucets. If, at such times, the hot water is turned on in a small cold bathroom, and is allowed to run until the tub is well filled, vapor condenses on windows, mirrors, and walls, and the cold room becomes perceptibly warmer. The heat given out by the condensing steam passes into the surrounding air and warms the room.
There is, however, another reason for the rise in temperature. If a large pail of hot soup is placed in a larger pail of cold water, the soup will gradually cool and the cold water will gradually become warmer. A red-hot iron placed on a stand gradually cools, but warms the stand. A hot body loses heat so long as a cooler body is near it; the cold object is heated at the expense of the warmer object, and one loses heat and the other gains heat until the temperature of both is the same. Now the hot water in the tub gradually loses heat and the cold air of the room gradually gains heat by convection, but the amount given the room by convection is relatively small compared with the large amount set free by the condensing steam.
25. Distillation. If impure, muddy water is boiled, drops of water will collect on a cold plate held in the path of the steam, but the drops will be clear and pure. When impure water is boiled, the steam from it does not contain any of the impurities because these are left behind in the vessel. If all the water were allowed to boil away, a layer of mud or of other impurities would be found at the bottom of the vessel. Because of this fact, it is possible to purify water in a very simple way. Place over a fire a large kettle closed except for a spout which is long enough to reach across the stove and dip into a bottle. As the liquid boils, steam escapes through the spout, and on reaching the cold bottle condenses and drops into the bottle as pure water. The impurities remain behind in the kettle. Water freed from impurities in this way is called distilled water, and the process is called distillation (Fig. 19). By this method, the salt water of the ocean may be separated into pure drinking water and salt, and many of the large ocean liners distill from the briny deep all the drinking water used on their ocean voyages.
Commercially, distillation is a very important process. Turpentine, for example, is made by distilling the sap of pine trees. Incisions are cut in the bark of the long-leaf pine trees, and these serve as channels for the escape of crude resin. This crude liquid is collected in barrels and taken to a distillery, where it is distilled into turpentine and rosin. The turpentine is the product which passes off as vapor, and the rosin is the mass left in the boiler after the distillation of the turpentine.
26. Evaporation. If a stopper is left off a cologne bottle, the contents of the bottle will slowly evaporate; if a dish of water is placed out of doors on a hot day, evaporation occurs very rapidly. The liquids which have disappeared from the bottle and the dish have passed into the surrounding air in the form of vapor. In Section 20, we saw that water could not pass into vapor without the addition of heat; now the heat necessary for the evaporation of the cologne and water was taken from the air, leaving it slightly cooler. If wet hands are not dried with a towel, but are left to dry by evaporation, heat is taken from the hand in the process, leaving a sensation of coolness. Damp clothing should never be worn, because the moisture in it tends to evaporate at the expense of the bodily heat, and this undue loss of heat from the body produces chills. After a bath the body should be well rubbed, otherwise evaporation occurs at the expense of heat which the body cannot ordinarily afford to lose.
Evaporation is a slow process occurring at all times; it is hastened during the summer, because of the large amount of heat present in the atmosphere. Many large cities make use of the cooling effect of evaporation to lower the temperature of the air in summer; streets are sprinkled not only to lay the dust, but in order that the surrounding air may be cooled by the evaporation of the water.
Some thrifty housewives economize by utilizing the cooling effects of evaporation. Butter, cheese, and other foods sensitive to heat are placed in porous vessels wrapped in wet cloths. Rapid evaporation of the water from the wet cloths keeps the contents of the jars cool, and that without expense other than the muscular energy needed for wetting the cloths frequently.
27. Rain, Snow, Frost, Dew. The heat of the sun causes constant evaporation of the waters of oceans, rivers, streams, and marshes, and the water vapor set free by evaporation passes into the air, which becomes charged with vapor or is said to be humid. Constant, unceasing evaporation of our lakes, streams, and pools would mean a steady decrease in the supply of water available for daily use, if the escaped water were all retained by the atmosphere and lost to the earth. But although the escaped vapor mingles with the atmosphere, hovering near the earth's surface, or rising far above the level of the mountains, it does not remain there permanently. When this vapor meets a cold wind or is chilled in any way, condensation takes place, and a mass of tiny drops of water or of
