Physics Part 34

2. What are two advantages in the expansion of water while freezing?

3. How much heat will be required to melt 1000 g. of ice and warm the water to 20C.?

4. How many grams of ice at 0C. can be melted by 400 g. of water at 55C.?

5. What are two advantages of the high specific heat of water? Two disadvantages?

6. If the specific heat of iron is 0.1125, how much ice at 0C. can be melted by a 200-g. ball of iron heated to 300C?

7. What is the temperature of a hot ball of iron weighing 80 g., if when placed on a piece of ice at 0C. it melts 90 g. of ice?

8. If 500 g. of copper at 400C. are placed into 3000 g. of water at 10C. what will be the resulting temperature?

9. What weight of water at 90C. will just melt 10 kg. of ice at 0C.?

10. If the smooth dry surface of two pieces of ice are pressed together for a short time the two pieces will be frozen into one piece. Explain.

11. Tubs of hot water are sometimes placed in vegetable cellars to prevent the vegetables from freezing. Explain.

12. How many B.t.u. are given out when 2 lbs. of water freeze?

(2) HEAT AND CHANGE OF STATE

[Ill.u.s.tration: FIG. 153.--The black cube in the upper corner represents one cubic inch of water. The entire cube represents the s.p.a.ce occupied by the cubic inch of water in the form of steam. The reduced s.p.a.ces at the bottom and sides show how much short the cube is of being one cubic foot. (American Radiator Co.)]

=184. Heat of Vaporization.=--In our study of evaporation in Art. 174 we considered the more rapidly moving or vibrating molecules in the liquid escaping to the air above and the slower moving molecules being left behind in the liquid; this means that a loss of heat will result upon evaporation, the liquid remaining becoming cooler as the process continues. Now just as a ball thrown up in the air loses its kinetic energy as it rises, and acquires energy of position or potential energy, so molecules escaping from a liquid lose a certain amount of kinetic energy or heat and acquire a corresponding amount of _energy of position_ or potential energy. _Conversely_, as the ball returns to the ground its potential energy is changed to kinetic energy. Similarly when vapor molecules return to the liquid condition they lose their energy of position and acquire kinetic energy. In other words, when a liquid evaporates a certain amount of heat disappears, or becomes _latent_ and when the vapor condenses the heat reappears, or becomes _sensible_ heat.

_The amount of heat that disappears when 1 g. of a substance is vaporized is called the heat of vaporization._ In the case of water at its boiling point, 536 calories of heat disappear when 1 g. of water turns to vapor, and this same amount of heat reappears when the vapor condenses.

The change of volume of water on turning to steam is shown in Fig. 153.

[Ill.u.s.tration: FIG. 154.--Effect of pressure on the boiling point.]

=185. The Boiling Point.=--The boiling temperature depends upon the pressure. The boiling point may be defined as _the temperature at which bubbles of vapor are formed within the liquid_. These bubbles increase the surface at which evaporation can take place in the liquid, and the princ.i.p.al reason why rapid application of heat to a liquid does not raise its temperature above the boiling point is that as more heat is applied more bubbles form so that the increase of evaporating surface supplies a correspondingly greater surface for cooling. The variation of the boiling temperature with changing pressure may be shown by partly filling a strong 7/8-in. test-tube with water. Close the neck with a one-hole rubber stopper through which pa.s.ses a gla.s.s tube to which is attached a soft rubber tube. (See Fig. 154.) Support the tube by a holder, heat the water and boil until all the air is driven from the tube, then close the soft rubber tube with a pinch c.o.c.k and hold the tube in an inverted position. On cooling the end of the tube above the water with cold water or snow, the vapor within is condensed and the pressure upon the water is reduced. Vigorous boiling begins at once. By condensing the vapor repeatedly the water may be made to boil at the room temperature. At the top of Mt. Blanc water boils at 84C. While in steam boilers at 225 lbs. pressure to the square inch the boiling point is nearly 200C.

=186. Laws of Boiling.=--The following statements have been found by experiments to be true.

1. Every liquid has its own _boiling_ point which under the same conditions of _pressure_ is always the same.

2. The temperature of the boiling liquid remains at the boiling point until all the liquid is changed into vapor.

3. The boiling point rises with increased pressure and falls if the pressure is diminished.

4. A boiling liquid and the vapor formed from it have the same temperature. On cooling, a vapor will liquefy at the boiling point.

[Ill.u.s.tration: FIG. 155.--Distilling apparatus.]

5. The solution of solid substances in a liquid raises its boiling point, additional energy being needed to overcome the adhesion involved in the solution. The boiling point is also affected by the character of the vessel containing the liquid. In gla.s.s the boiling point is 101.

[Ill.u.s.tration: FIG. 156.--A vacuum pan.]

=187. Distillation of Water.=--Usually when solids are dissolved in liquids the vapor coming from the liquid contains none of the dissolved solid. Thus by evaporating salt sea water, and collecting and condensing the vapor, pure water is obtained. _Distillation_ is the process of boiling a liquid and condensing the vapor formed back again into a liquid. (See Fig. 155.) The liquid to be distilled is placed in vessel _F_ and boiled. The vapor is conducted into the tube _J_ which is surrounded by a larger tube containing cold water. The vapor is condensed on the cold walls of the tube. The resulting liquid is collected in the vessel _R_. Distillation is employed for two purposes: (a) To remove impurities from a liquid (water is purified in this way).

(b) Mixtures of different liquids having different boiling points may be separated by distillation. The one having the lower boiling point will be vaporized first. Thus a mixture of alcohol and water, on distillation yields a distillate having a much larger percentage of alcohol than at first. Repeating this process which is called _fractional distillation_ yields alcohol of increasing strength of purity. Distilled liquor such as alcohol, brandy, and whisky are made by distilling fermented liquor, alcohol being made from fermented grains. Gasoline and kerosene are distilled from crude petroleum. Sometimes as in the production of sugar or evaporated milk the object is to remove the water by evaporation in order to obtain the solid material. Since the two substances named are injured by heating, the syrup, or milk is evaporated under reduced pressure in a _vacuum pan_, that is in a boiler from which air and vapor are removed by an air pump. (See Fig. 156.)

=188. Artificial Cooling.=--The fact has been brought out that when a solid is melted, a certain amount of heat, called the heat of fusion, is absorbed or disappears. This absorption of heat is also noticed when a solid is liquefied by dissolving it in a liquid as well as when it is liquefied by simply applying heat. Thus if some table salt is placed in a tumbler of water the temperature of the solution is lowered several degrees below that of the salt and water used. The liquefaction or solution of the salt has been accompanied by an absorption or disappearance of heat. This heat has been taken from the salt and from the water, resulting in a lowered temperature. Sal ammoniac or ammonium nitrate when dissolved in water produce a much more marked cooling effect than does table salt. The dissolving of a crystal in a liquid is something like evaporation, except that the molecules of the liquid attract the molecules of the solid and thus a.s.sist the change of state.

=189. Freezing Mixtures.=--If one attempts to freeze a solution of salt and water, ice will not form at 0C. but several degrees lower. The ice formed however is pure. Evidently the attraction of the molecules of salt for the water molecules prevented the formation of ice until the motions of the water molecules had been reduced more than is necessary in pure water. As the temperature of freezing water is that of melting ice, ice in a salt solution melts at lower temperature than in pure water. In a saturated salt solution this temperature is -22C. It is for this reason that the mixture of ice and salt used in freezing cream is so effective, the salt water in melting the ice, being cooled to a temperature many degrees below the freezing point of the cream. The best proportion for a freezing mixture of salt and ice is one part salt to three parts of finely powdered or shaved ice.

=190. Refrigeration by Evaporation.=--Intense cold is also produced by permitting the rapid evaporation of liquids under pressure. Carbon dioxide under high pressure is a liquid, but when allowed to escape into the air evaporates so rapidly that a portion of the liquid is frozen into solid carbon dioxide which has a temperature of -80 C. The evaporation of liquid ammonia by permitting it to escape into a pipe, under reduced pressure, is used on a large scale as a means of producing cold in cold storage and refrigeration plants. (See Fig. 157.)

[Ill.u.s.tration: FIG. 157.--Diagram of a refrigerating system.]

The essential parts of the refrigerating system employing ammonia is represented in Fig. 157. The _compressor_ exhausts ammonia gas from the coiled pipe in "_E_" and compresses the gas in "_C_," where under 150 pounds pressure and the cooling effect of water it condenses to liquid ammonia. This is allowed to pa.s.s slowly through the regulating valve, whereupon it evaporates and expands in the long coiled pipe in "_E_" on its way back to the compressor. This evaporation and expansion causes a large amount of heat to be absorbed from the brine, cooling the latter below the freezing point of pure water and thus permitting the freezing of cans of water suspended in the brine. The chilled brine may also be sent through pipes in order to cool storage rooms containing meat or other food products. The ammonia absorbs heat when it vaporizes and gives up heat when it is compressed and liquified.

Important Topics

1. Heat of vaporization, of water 536 calories per gram.

2. Boiling point, effect of pressure upon boiling point, laws of boiling.

3. Distillation, artificial cooling, freezing mixtures, refrigeration by evaporation.

Exercises

1. How much heat is required (a) to melt 1 g. of ice at 0C., (b) to raise the temperature of the water resulting to 100C., (c) to change this water to steam?

2. If the water leaving a steam radiator is as hot as the steam how is the room warmed?

3. What is the effect of placing salt upon icy sidewalks in cold weather?

4. Is rain water distilled water? Is it perfectly pure?

5. What are two advantages of the high heat of vaporization of water?

6. If the heat from 1 g. of steam at 100C. in changing to water and cooling to 0C. could be used in melting ice at 0C. how much ice would be melted?

(3) HEAT AND WORK

=191. Necessity for Heat Energy.=--From early times man has been able to transform motion into heat, and has used this ability in many directions as in starting fires and warming himself by friction. It took man many centuries, however, to devise an effective machine for transforming heat into mechanical energy or to use it in doing work.

The _power_ of a man is small and as long as the work of the world had to be done by man power, progress was r.e.t.a.r.ded. When man began the use of beasts of burden, he took a long step in advance since one man could then employ and direct the power of many men in the animals he controlled. Man also built water-wheels and windmills thus gaining power directly from the forces of nature and these added much to his working ability. But he took the greatest step in gaining control over his surroundings when he learned to use heat energy and to make it drive his machines.

=192. Heat Engines.=--At the present time there is a great variety of _heat engines_ in use such as _steam_, _hot air_, _gas_, and _gasoline_ engines, all using _heat energy_ to produce motion. The expansive power of steam when confined has been observed for hundreds of years and many different machines have been invented to use it in doing work.

[Ill.u.s.tration: FIG. 158.--Cross-section view of cylinder and steam chest of a steam engine.]