General Science, Bertha May Clark [miss read books TXT] 📗

larger quantity will become only lukewarm. Both vessels receive the same amount of heat from the sun, but in one case the heat is utilized in heating to a high temperature a small quantity of water, while in the second case the heat is utilized in warming to a lower degree a larger quantity of water. Equal amounts of heat do not necessarily produce equivalent temperatures, and equal temperatures do not necessarily indicate equal amounts of heat. It takes more heat to raise a gallon of water to the boiling point than it does to raise a pint of water to the boiling point, but a thermometer would register the same temperature in the two cases. The temperature of boiling water is 100° C. whether there is a pint of it or a gallon. Temperature is independent of the quantity of matter present; but the amount of heat contained in a substance at any temperature is not independent of quantity, being greater in the larger quantity.

15. The Unit of Heat. It is necessary to have a unit of heat just as we have a unit of length, or a unit of mass, or a unit of time. One unit of heat is called a calorie, and is the amount of heat which will change the temperature of 1 gram of water 1° C. It is the amount of heat given out by 1 gram of water when its temperature falls 1° C., or the amount of heat absorbed by 1 gram of water when its temperature rises 1° C. If 400 grams of water are heated from 0° to 5° C., the amount of heat which has entered the water is equivalent to 5 × 400 or 2000 calories; if 200 grams of water cool from 25° to 20° C., the heat given out by the water is equivalent to 5 × 200 or 1000 calories.

16. Some Substances Heat more readily than Others. If two equal quantities of water at the same temperature are exposed to the sun for the same length of time, their final temperatures will be the same. If, however, equal quantities of different substances are exposed, the temperatures resulting from the heating will not necessarily be the same. If a basin containing 1 lb. of mercury is put on the fire, side by side with a basin containing an equal quantity of water, the temperatures of the two substances will vary greatly at the end of a short time. The mercury will have a far higher temperature than the water, in spite of the fact that the amount of mercury is as great as the amount of water and that the heat received from the fire has been the same in each case. Mercury is not so difficult to heat as water; less heat being required to raise its temperature 1° than is required to raise the temperature of an equal quantity of water 1°. In fact, mercury is 30 times as easy to heat as water, and it requires only one thirtieth as much fire to heat a given quantity of mercury 1° as to heat the same quantity of water 1°.

17. Specific Heat. We know that different substances are differently affected by heat. Some substances, like water, change their temperature slowly when heated; others, like mercury, change their temperature very rapidly when heated. The number of calories needed by 1 gram of a substance in order that its temperature may be increased 1° C. is called the specific heat of a substance; or, specific heat is the number of calories given out by 1 gram of a substance when its temperature falls 1° C. For experiments on the determination of specific heat, see Laboratory Manual.

Water has the highest specific heat of any known substance except hydrogen; that is, it requires more heat to raise the temperature of water a definite number of degrees than it does to raise the temperature of an equal amount of any other substance the same number of degrees. Practically this same thing can be stated in another way: Water in cooling gives out more heat than any other substance in cooling through the same number of degrees. For this reason water is used in foot warmers and in hot-water bags. If a copper lid were used as a foot warmer, it would give the feet only.095 as much heat as an equal weight of water; a lead weight only.031 as much heat as water. Flatirons are made of iron because of the relatively high specific heat of iron. The flatiron heats slowly and cools slowly, and, because of its high specific heat, not only supplies the laundress with considerable heat, but eliminates for her the frequent changing of the flatiron.

18. Water and Weather. About four times as much heat is required to heat a given quantity of water one degree as to heat an equal quantity of earth. In summer, when the rocks and the sand along the shore are burning hot, the ocean and lakes are pleasantly cool, although the amount of heat present in the water is as great as that present in the earth. In winter, long after the rocks and sand have given out their heat and have become cold, the water continues to give out the vast store of heat accumulated during the summer. This explains why lands situated on or near large bodies of water usually have less variation in temperature than inland regions. In the summer the water cools the region; in the winter, on the contrary, the water heats the region, and hence extremes of temperature are practically unknown.

19. Sources of Heat. Most of the heat which we enjoy and use we owe to the sun. The wood which blazes on the hearth, the coal which glows in the furnace, and the oil which burns in the stove owe their existence to the sun.

Without the warmth of the sun seeds could not sprout and develop into the mighty trees which yield firewood. Even coal, which lies buried thousands of feet below the earth's surface, owes its existence in part to the sun. Coal is simply buried vegetation,—vegetation which sprouted and grew under the influence of the sun's warm rays. Ages ago trees and bushes grew "thick and fast," and the ground was always covered with a deep layer of decaying vegetable matter. In time some of this vast supply sank into the moist soil and became covered with mud. Then rock formed, and the rock pressed down upon the sunken vegetation. The constant pressure, the moisture in the ground, and heat affected the underground vegetable mass, and slowly changed it into coal.

The buried forest and thickets were not all changed into coal. Some were changed into oil and gas. Decaying animal matter was often mixed with the vegetable mass. When the mingled animal and vegetable matter sank into moist earth and came under the influence of pressure, it was slowly changed into oil and gas.

The heat of our bodies comes from the foods which we eat. Fruits, grain, etc., could not grow without the warmth and the light of the sun. The animals which supply our meats likewise depend upon the sun for light and warmth.

The sun, therefore, is the great source of heat; whether it is the heat which comes directly from the sun and warms the atmosphere, or the heat which comes from burning coal, wood, and oil.

CHAPTER III

OTHER FACTS ABOUT HEAT

20. Boiling. Heat absorbed in Boiling. If a kettle of water is placed above a flame, the temperature of the water gradually increases, and soon small bubbles form at the bottom of the kettle and begin to rise through the water. At first the bubbles do not get far in their ascent, but disappear before they reach the surface; later, as the water gets hotter and hotter, the bubbles become larger and more numerous, rise higher and higher, and finally reach the surface and pass from the water into the air; steam comes from the vessel, and the water is said to boil. The temperature at which a liquid boils is called the boiling point.

While the water is heating, the temperature steadily rises, but as soon as the water begins to boil the thermometer reading becomes stationary and does not change, no matter how hard the water boils and in spite of the fact that heat from the flame is constantly passing into the water.

If the flame is removed from the boiling water for but a second, the boiling ceases; if the flame is replaced, the boiling begins again immediately. Unless heat is constantly supplied, water at the boiling point cannot be transformed into steam.

The number of calories which must be supplied to 1 gram of water at the boiling point in order to change it into steam at the same temperature is called the heat of vaporization; it is the heat necessary to change 1 gram of water at the boiling point into steam of the same temperature.

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