Questions 1. Why are the bolts of an iron bridge put in hot? 2. What takes place in the water of a pond as the weather grows colder in the fall? 3. Suppose a copper wire is fused into a glass tube. What will happen when they cool down to the temperature of the air? 4. Does the same thing happen if a platinum wire is used? Explain. 5. What will happen if the corner of a piece of ice is held in the top of a beaker of warm water? The result can be seen more readily if the water is slightly colored. 6. What takes place in a beaker of water that is being heated over a Bunsen burner if the burner is not under the middle of the beaker? 7. Is a football tighter in warm weather or in cold? Which is better, to fill the ball by blowing with the breath or to use an air pump? Why? 8. Why does decreasing the pressure on a given quantity of gas increase its volume? 9. Why does heating it have the same effect? 10. Why do water pipes burst when the water in them freezes? 11. Would increasing the pressure on ice at 0° C. melt it if the water did not expand on freezing? 12. What is the effect of placing a beaker of water at 80° C. under the receiver of an air pump and then exhausting the air? 13. Why does moisture collect on a window pane in cool weather? 14. Why is sea water distilled before being used in the boilers of a steamship? 15. Why does evaporation take place more slowly when the air is moist? 16. Why do wet clothes freeze dry on a cold day in winter? 17. Why does a wet road dry sooner if the wind blows? 18. Why are the cans, containing water to be frozen in an ice machine, placed in brine instead of in water? 19. Describe and explain the action that takes place in each part of the refrigerating machine shown in Fig. 256 and state the reason for the direction of flow in the pipes. Problems 1. How much will a steel rail 30 ft. long increase in length when the temperature changes from zero to 22° C.? 2. If a metal rod 150 cm. long at 0° C. expands 0.24 cm. on being heated to 100° C., what is its coefficient of linear expansion? 3. How much would a meter scale made of invar expand on being heated from 20° C. to + 28° C.? 4. The density of ice is 0.917. What will be the volume of a cubic foot of water after freezing? 5. A balloon contains 4000 cu. ft. of gas at 23° C. What will be the volume of the gas at 0° C.? 6. If the volume of an inclosed body of air is 426 c.c. at zero what will it be when the temperature is raised to 20° C., the pressure being the same? 7. If the volume of a gas at 21° C. and a pressure of 950 mm. of mercury is 126 c.c., what will it be at 0° C. and a pressure of 760 mm. ? 8. The pressure per sq. cm. on 150 c.c. of gas at 16° C. is 3.2 kg. What must it be to reduce the volume to 120 c.c. when the temperature is 23° C.? 9. The temperature of 200 c.c. of gas at a pressure of 760 mm. of mercury is 21° C. What will it be if the volume is reduced to 150 c.c. by a pressure of 1140 mm. ? 10. What would the temperature have been in problem 9, if the pressure used to reduce the volume of the gas had been two atmospheres, or 1520 mm.? 11. Which contains the greater amount of moisture, air at 15° C. having a humidity of 90% or air at 23° C. with a humidity of 60%? 12. How much moisture condenses from a cubic meter of air that is saturated at 20° C. when the temperature falls to 8° C.? 13. On a day when the temperature of the air was 20° C. the dewpoint was found to be 11° C. What was the relative humidity? 14. If the humidity is 80% and the temperature of the dew-point is 16° C., how much moisture is in the air per cubic meter? How much would it hold if saturated, and what was the temperature of the air? IV. CALORIMETRY 302. The Measurement of Heat. In order to measure the quantity of heat that is given to a certain amount of water, two things must be considered: the mass of the water and the change of the temperature. Since there are different units of mass and different thermometric scales, several thermal units are possible. The two most important are defined as follows: the quantity of heat required to raise 1 g. of water through 1o C. is called a calorie; the quantity of heat required to raise 1 lb. of water through 1° F. is called a British thermal unit (B. T. U.). 1 B. T. U. 252 calories. The measurement of the heat used in changing either the temperature or the physical condition of a body is called calorimetry. = 303. Specific Heat. Demonstration. — Place 50 g. of shot in one test tube and 50 g. of iron filings in another similar tube. Raise both to the same temperature by placing them in a beaker of hot water. Into each of two small beakers pour 100 g. of water that has been cooled to the zero point by means of ice. Pour the shot into one beaker and the filings into the other. Take the temperature of the water in each, and it will be found that the filings have given to the water the greater amount of heat, as is shown by the higher temperature in the beaker containing them. This demonstration shows that iron has a greater amount of heat than lead at the same temperature. It follows that more heat would be needed to raise 50 g. of iron 1° in temperature than to raise 50 g. of lead 1°. The ratio between the quantity of heat required to raise the temperature of a certain mass of any substance one degree, and the quantity of heat required to raise the same mass of water 1 French engineers also use a calorie 1000 times as great; i.e., 1 kg. of water is the basis instead of 1 g. one degree, is the specific heat of that substance; or, the specific heat of a substance is the number of calories required to change the temperature of 1 g. of that substance 1° C. A convenient 304. The Measurement of Specific Heat. method of measuring the specific heat of a body is the method of mixtures. This depends upon the fact that when two bodies that are at different temperatures are put together, the temperature of one will fall and that of the other will rise until they have reached the same temperature. It also depends upon the principle that the heat absorbed by the cool body in heating is exactly the amount given out by the hot body in cooling. This principle is fundamental in all work in specific heat and may be stated in its simplest form as follows: Heat gained Heat lost. The quantity of heat absorbed by the cool body in heating = mass X change in temperature X specific heat. The quantity of heat given out by the hot body in cooling = mass change in temperature X specific heat. That is, = EXAMPLE. Two pounds of fine shot at 90° were poured into one pound of water at 15°, and the resulting temperature was 20°. What was the specific heat of the shot? From Formula 50, since the specific heat of water = whence s = 305. Water Equivalent. It is evident that in all similar measurements, account should be taken of the change in temperature of the containing beaker, or calorimeter. In order to do this it is necessary to find its water equivalent, or the mass of water that will require the same number of calories to raise its temperature one degree as the beaker requires. Numerically this is the product of the mass of the beaker by its specific heat. EXAMPLE. Ice weighing 50 g. was put into a 20 g. aluminum beaker containing 300 g. of water at 60° C. What was the temperature of the resulting mixture? 306. Heat of Fusion. If heat is applied to a beaker of crushed ice, it will be noticed that while the ice is being melted the temperature of the resulting water is the same as that of the ice, i.e., zero. The effect of the heat is not to change the temperature, but to change the physical state from solid to liquid. The number of heat units required to melt a unit mass of a substance without raising its temperature is called the heat of fusion of the substance. On solidification, the same amount of heat is given out. Demonstration. Pour 500 g. of water at 80° C. into a glass beaker weighing 100 g., and into this put 150 g. of cracked ice as dry as possible. Stir the ice until it is melted and take the resulting temperature of the water. It will be found to be about 43.8° C. The heat of fusion of ice can now be calculated as follows: The heat given out by the 500 g. of water and by the 100 g. of |