Make sure to consult the teacher's handbook, Inquiry in Action - Investigating Matter Through Inquiry, for two demonstrations that can be done to introduce solubility and measuring crystal mass before having the class conduct this investigation. Physical scientists mass equal amounts of the five crystals that they have already been working with. Then they dissolve each in water as a step closer to identifying the unknown crystal in the group. Ideally, you would immediately follow this with the final activity in the mini unit.