21. The Amount of Heat Absorbed. The amount of heat which must be constantly supplied to water at the boiling point in order to change it into steam is far greater than we realize. If we put a beaker of ice water (water at 0° C.) over a steady flame, and note (1) the time which elapses before the water begins to boil, and (2) the time which elapses before the boiling water completely boils away, we shall see that it takes about 5-1/4 times as long to change water into steam as it does to change its temperature from 0° C. to 100° C. Since, with a steady flame, it takes 5-1/4 times as long to change water into steam as it does to change its temperature from 0° C. to the boiling point, we conclude that it takes 5-1/4 times as much heat to convert water at the boiling point into steam as it does to raise it from the temperature of ice water to that of boiling water.
The amount of heat necessary to raise the temperature of 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.