Termodinamica

A Simple and Economic Three-Dimensional Model for the PWT Surface of Water
Juan A. Coch Frugoni, Marilene Zepka, Alvaro Rocha Figueira, and Miriam Coretti Universidade de Rio Grande, Rio GrandeR.S., Brasil
The students in our physical chemistry courses have difficulties understandine three-dimensional models when. as is usual, they are presented as drawings. In order to facilitate betterunderstandine in the studv of phase equilibrium of Dure subtances, we use aUtrulythree~di&nsion~l model (see%ig. 1).In the course each student builds hisher own model by the following procedure:
1) Cutthe patterns shown in Figure 2 from cardhoard. 2) FoUowinethe foldine instructions of F i..m e 2..make Unib 1(Fie. 3). 1 and'i (Fig. 4). i ( ~ i gSA,,and1 (Fig. 5Hj. tln order to i i . pmve thecurvature of ihr surfaces, n,ll them around a pencil

before gluing.)
3) Join with glue Units 1and 2 with Unit 3. 4) Add Unit 4 to complete model. 5) Paint the different surfaces. The surfaces representingtwo phases in equilibrium, for example L-V (blue and yellow, respectively),should be painted with the color representing the

mixture (green). This model is easy to construct and very economical,and it shows clearly the conditions for existence of the phases and the conditions for phase changes. Students have found the model to be very instructive even though i t is not to scale. Variations ofthis technique could be used t o produce other three-dimensional models for chemistry courses.

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Figure 2. The patterns uJed in the construction of lhe P W madel for water. Observecutting, folding, and gluing inshuctions.

Figure 1. The cardboard model for lhe PWsurface of water. The colors of the
phases inlhis model are blue (Vand G), yellow (L), geen (Land V), red (S), mnge(Sand L), and purple (S and V).

Flgure 3. Tha cut a 3 folded Unit 1. m

Figure 4. Units 1 and 2 assembled.

Figure 5. Unhs 3 and 4 ready for assembly.

1048

Journal of Chemical...