Some Lecture Demonstrations' FREDERIC B. D W O N 2 Baldwin-Wallace College, Bcrea, Ohio 1. BOYLE'S LAW OST apparatus for the demonstration of Boyle's law requires the observation of volumes of gas in a small tube visible to the demonstrator but not to the student in the last row. The apparatus herein described is designed to demonstrate visibly to each member of the class the principle as simply and forcefully as possible.
ume has been halved and the pressure doubled. If the final step were to be taken and the third flask filled with water the system would consist of one liter of air a t a pressure of four atmospheres. This last step is not
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The apparatus shown in Figure 1consists of four oneliter flasks fitted with two-hole rubber stoppers wired in place, and is connected as shown with an open mercurial U-shaped manometer. The connections are of 7-mm. glass tubing. In use, the system is evacuated with a pump (Hyvac or filter pump) connected a t P to demonstrate atmospheric pressure (Figure 2). Comparisons may be made with the usual types of barometers. Air is then admitted by disconnecting the pump. The system now consists of four liters of air under a pressure of one atmosphere (Figure 1). A connection is now made with the water supply a t P and water admitted until the f i s t flask is full (Figure 3). The system now contains three liters of air under a pressure of one-third of an atmosphere more than the air in the surrounding room or one and one-third atmospheres. In a similar manner the second flask is filled with water a;nd the system then contains two liters of air under a pressure of two atmospheres (one more than normal atmospheric pressure) (Figure 4). It is immediately apparent that the vol-
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Presented before the Division of Chemical Education at the ninety-sixth meeting of the A. C. s., Milwaukee, Wisconsin, September 7, 1938. Present address: Eaton Rapids. Michigan.
advisable because it is diicult to wire the stoppers in securely enough to withstand the pressure; the manometer must be made too high for convenience in handling; and there is danger from flying glass if the flask
should be b r ~ k e n . I~f this last step is taken a shield "he author has repeatedly subjected s e w a l flasks to a pressure in excess of five atmospheres without once having a flask break. When a flask w a s purposely broken with a hammer a forceful explosion took place.
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should be provided to protect the class and demonstrator if the flask should fail. Many variations will suggest themselves. If the demonstrator prefers, a closed tube manometer may be used, in which case it must be correspondingly taller. The demonstration may be repeated starting with onehalf atmosphere of air in the four flasks. It should be emphasized that the pressure indicated by an open-end manometer is in addition to or above the atmospheric pressure. A measuring stick is used which is calibrated in thirds of an atmosphere (25, 50, 75, and 100 cm.) and may be compared with the usual meter stick to convert to mm. or cm. of mercury. I t should be
demonstration of change of state is accomplished with the aid of the apparatus shown in Figure 5. It is a modification of the Cryophorus shown in physics apparatus catalogs and consists of two ampoules drawn down from one-inch test tubes and connected with 7mm. glass tubing. A short side arm is sealed in as further pointed out that these are rough measurements to illustrate a principle. More precise technic would require corrections for the volume of tubing, calibration of flasks, partial pressure of water vapor, and would show the irregularities caused by a departure from the perfect gas law. The enumeration of these sources of error may be used as an assignment to interest those who have had previous training in chemistry or physics, while demonstrating the fundamental principle to those who have not had such training. With this apparatus all measurements are visible from any part of the lecture room and yield the following simple values, easily checked mentally by any student.
2. CHANGE OF STATE Several demonstrations of the transformations of energy accompanying changes of state have been described in this JOURNAL,' and elsewhere. A simple
' HOLMESAND SNODDY, J. CAEM.EDnC., 8,1166 (June, 1931); HA~EN did., , 9, 1115 (June, 1932); MARKLEY, ibid., 11, 251 (April, 1934).
indicated a t C. About 20 ml. of ether are introduced through the side arm and distributed equally between the two ampoules. The system is then partially evacuated by connecting to the aspirator or water pump. This causes the ether to boil violently, sweeping out the air. The boiling is continued for about a minute; then the rubber connection to the pump is closed with a screw clamp and the side arm is sealed off. In use, all of the ether is poured into one of the ampoules and the other surrounded either by liquid air or by an acetone-dry ice mixture. The ether vapor is condensed in the cold ampoule, the pressure in the system is reduced, and the liquid ether boils rapidly. Within a few minutes a thick layer of frost has formed .on the bottom of the ampoule containing the boiling ether, giving visual evidence that heat is being absorbed from the walls of the tube and the surrounding .atmosphere (Figure 6). Comparisons may be made between this simple de-
vice and the artificial refrigerator, a steam heating plant, or any distillation or evaporation process. In drawing an analogy to the artificial refrigerator it must be pointed out that to complete the cycle a pump (or absorption cycle) is used which reduces the pressure in the cooling unit A in the refrigerator box, compresses the gas which gives up its heat a t a higher temperature to the room through a radiator B, liquefies, and is returned to A thrbugh a valve. In a steam-heating plant, A represents the furnace boiler where heat is introduced, the liquid vaporized and passed to the radiators; B, where the heat is given to the room and the vapor condensed. The return in this case is either by gravity or a pump. This apparatus becomes a permanent piece of lecture demonstration equipment. It never needs adjustment, is instantly ready for service, and the demonstration is complete within three to five minutesless time than it takes to tell about it.
