Sphere-Model for the Solution of Iodine in Ethanol

Problem The particle model of matter may be introduced through the solution of two substances (see E4.6). Students will not come up independently with the particle interpretation - they need help by the verbal particle explanation or better by demonstrating model experiments. One can use one big sphere as a model for the iodine particle and one little sphere as a model for the ethanol particle. The model experiment can be done without many comments - one could then ask the students to associate certain aspects of the model with the original (see E4.6). In the model discussion it is important to expose the model traits, but also the irrelevant items. One can compare the model experiment with the model drawing (see Fig. 4.6). 

Material Large glass bowl (on the overhead projector) big and small cellulose spheres (i.e. 30 mm and 12 mm diameters respectively). 

Procedure Fill the surface of the large glass bowl up to three quarters with small spheres (models for ethanol particles). Create some space in the middle and place 16 or 25 big spheres arranged in a square shape (models for iodine particles in iodine crystal). Carefully shake the glass bowl until all spheres are mixed and have spread over the bowl. 

Observation At first the special arrangement of big spheres is observed, after shaking the bowl all moving spheres are spread uniformly over the entire surface of the bowl. 

Problem The P atom is neither white nor red, only after many P atoms have been arranged in a crystal that can be determined as white or red phosphorus. If 

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