simple science

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Simple Science

21. The Amount of Heat Absorbed
Other Facts about Heat:
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.
22. General Truths
Other Facts about Heat:
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
Other Facts about Heat:
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.

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.

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 Heat
Other Facts about Heat:
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
Other Facts about Heat:
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. 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.

FIG. - In order that the steam which passes through the coiled tube may be quickly cooled and condensed, cold water is made to circulate around the coil. The condensed steam escapes at w.
26. Evaporation
Other Facts about Heat:
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. We know 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
Other Facts about Heat:
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 small particles of snow is formed. When these drops or particles become large enough, they fall to the earth as rain or snow, and in this way the earth is compensated for the great loss of moisture due to evaporation. Fog is formed when vapor condenses near the surface of the earth, and when the drops are so small that they do not fall but hover in the air, the fog is said "not to lift" or "not to clear."

If ice water is poured into a glass, a mist will form on the outside of the glass. This is because the water vapor in the air becomes chilled by contact with the glass and condenses. Often leaves and grass and sidewalks are so cold that the water vapor in the atmosphere condenses on them, and we say a heavy dew has formed. If the temperature of the air falls to the freezing point while the dew is forming, the vapor is frozen and frost is seen instead of dew.

The daily evaporation of moisture into the atmosphere keeps the atmosphere more or less full of water vapor; but the atmosphere can hold only a definite amount of vapor at a given temperature, and as soon as it contains the maximum amount for that temperature, further evaporation ceases. If clothes are hung out on a damp, murky day they do not dry, because the air contains all the moisture it can hold, and the moisture in the clothes has no chance to evaporate. When the air contains all the moisture it can hold, it is said to be saturated, and if a slight fall in temperature occurs when the air is saturated, condensation immediately begins in the form of rain, snow, or fog. If, however, the air is not saturated, a fall in temperature may occur without producing precipitation. The temperature at which air is saturated and condensation begins is called the dew point.
28. How Chills are Caused
Other Facts about Heat:
The discomfort we feel in an overcrowded room is partly due to an excess of moisture in the air, resulting from the breathing and perspiration of many persons. The air soon becomes saturated with vapor and cannot take away the perspiration from our bodies, and our clothing becomes moist and our skin tender. When we leave the crowded "tea" or lecture and pass into the colder, drier, outside air, clothes and skin give up their load of moisture through sudden evaporation. But evaporation requires heat, and this heat is taken from our bodies, and a chill results.

Proper ventilation would eliminate much of the physical danger of social events; fresh, dry air should be constantly admitted to crowded rooms in order to replace the air saturated by the breath and perspiration of the occupants.
29. Weather Forecasts
Other Facts about Heat:
When the air is near the saturation point, the weather is oppressive and is said to be very humid. For comfort and health, the air should be about two thirds saturated. The presence of some water vapor in the air is absolutely necessary to animal and plant life. In desert regions where vapor is scarce the air is so dry that throat trouble accompanied by disagreeable tickling is prevalent; fallen leaves become so dry that they crumble to dust; plants lose their freshness and beauty.

The likelihood of rain or frost is often determined by temperature and humidity. If the air is near saturation and the temperature is falling, it is safe to predict bad weather, because the fall of temperature will probably cause rapid condensation, and hence rain. If, however, the air is not near the saturation point, a fall in temperature will not necessarily produce bad weather.

The measurement of humidity is of far wider importance than the mere forecasting of local weather conditions. The close relation between humidity and health has led many institutions, such as hospitals, schools, and factories, to regulate the humidity of the atmosphere as carefully as they do the temperature. Too great humidity is enervating, and not conducive to either mental or physical exertion; on the other hand, too dry air is equally harmful. In summer the humidity conditions cannot be well regulated, but in winter, when houses are artificially heated, the humidity of a room can be increased by placing pans of water near the registers or on radiators.
30. Heat Needed to Melt Substances
Other Facts about Heat:
If a spoon is placed in a vessel of hot water for a few seconds and then removed, it will be warmer than before it was placed in the hot water. If a lump of melting ice is placed in the vessel of hot water and then removed, the ice will not be warmer than before, but there will be less of it. The heat of the water has been used in melting the ice, not in changing its temperature.

If, on a bitter cold day, a pail of snow is brought into a warm room and a thermometer is placed in the snow, the temperature rises gradually until 32 F. is reached, when it becomes stationary, and the snow begins to melt. If the pail is put on the fire, the temperature still remains 32F., but the snow melts more rapidly. As soon as all the snow is completely melted, however, the temperature begins to rise and rises steadily until the water boils, when it again becomes stationary and remains so during the passage of water into vapor.

We see that heat must be supplied to ice at 0 C. or 32 F. in order to change it into water, and further, that the temperature of the mixture does not rise so long as any ice is present, no matter how much heat is supplied. The amount of heat necessary to melt 1 gram of ice is easily calculated.

Heat must be supplied to ice to melt it. On the other hand, water, in freezing, loses heat, and the amount of heat lost by freezing water is exactly equal to the amount of heat absorbed by melting ice.

The number of units of heat required to melt a unit mass of ice is called the heat of fusion of water.


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