A Transparent Phase Rule Model LIU-SHENG TS'AI Yenching University, Peking, China TUDENTS in physical chemistry often find it difficult to visualize the phase rule relation from the diagrams given in the textbooks which are intended to represent a space relationship in three dimensions. To overcome this difficulty various solid models,l usually made of plaster, are constructed.
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plate as an isotherm. The plates are then arranged in the order of increasing temperatures. The threedimensional character of the system will then be clearly seen. Similarly, for a one-component system one may use the curves given by Verwiebel (thermodynamic surfaces of C0' or H20). For a two-component system, the well-known KCI-Hz0 system might be used. Curves on the temperature-pressure plane are drawn on each plate for each composition. Inasmuch as the model is primarily for demonstration purposes it is only necessary to sketch the curves roughly. Chinese inkJ and a Chinese brush can be used for this purpose. Figure 2 shows schematically the curves for the system KC1 and H'O. The solid space can be brought out more clearly both by increasing the number of plates and by coloring different regions with different colored inks.8 In the region where more than one solid appears (such as in a eutectic mixture) the area may be dotted with different colors to represent different solids. The plate on top of the model may be used for drawing the c u m representing any one composition-temperature plane. A single model such as here described may be used to represent different systems since the curves
The disadvantages of this type of model, however, are twofold. First, the construction of such a model is time-consuming, and second, the regions under the surface of the model are not visible. The present article describes a type of model which is easy to construct and, since i t consists entirely of glass plates, is transparent. Thick cellophane or pliofilm sheets now available are superior, but more expensive than the glass plates. Grooves as shown in Figure 1 are cut on a wooden X 1". These baseboard approximately 10" X grooves are of such dimensions that the glass plates can be held M y in place and yet may easily be removed if desired. The board may then be painted white. Eleven glass plates, each 10" X 101/*', and another, are prepared. The arrangement of these 10" X 1lW, plates is shown in Figure 1,the larger plate being placed in the back. The model may now be used to represent either a one-, two-, or three-component system. For constructing a three-component system, one may use, as an example, the figures given by Taylor.' The curves (tie-lines not included) are sketched on each
Temperature
F m w 2.-SKETCH ~ OP KCI-H.0 SYSTRP Each of the numhered curves is drawn on a separate plate nnd rrprrirnts the P-T curve fur each composi. tiun. l'heordcr is that of incrcajinw KCIconcentration. Curve 3 represents the eutectic composition
can be easily erased with a damp cloth and new curves drawn.
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Chinese ink can be easily prepared by grinding lampblack in a mortar with enough two per cent gelatin solution to give the proper consistency. Inks of different colors can be prepared in a similar way using colored pigments.
Am. Phys. Teacher, 3,180 (1935). 1 TAYLOR, Editor, "A treatise on physical chemistry," 2nd ed., D. Van Nostrand Ca.,Inc., New York City, 1931, p. 576. I
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