Hi, I am interested in letting high school students explore the effect of various conditions on the degree to which a crystallizing substance picks up impurities. (This is to establish groundwork for understanding how inclusions in zeolites are used as indicators conditions in the early earth). That might, for example, be growing a CuSO4 from a solution that includes traces of NiCl2, then identifying the amount of Ni in the crystal. We are working on three questions that we would welcome your input on: 1) What substances seem like promising ones to test? One should be relatively easy to crystallize (leading us to CuSO4). The other obviously should be a reasonable candidate for inclusion in the crystal, which we are assuming to be something near the other cation on the periodic table. 2) The second question is what variables seem likely to have an effect and are easy to control. Temperature and concentration leap to mind. Any ideas about others that might be reasonably checked? 3) How do we take he resulting crystals and determine the level of impurities? Absolute values values would be great, but I am expecting that relative values are more likely. We have access to spectrometers (the Vernier SpectoVis) that can read absorbance in a solution or through an LED. Would that likely pick up ppm impurities? Are there impurities that will give color changes?