In a reaction such as KI+Pb3(NO3)2--->PbI2+ K2NO3...how do you know the product PbI2 is a solid?

Answer 1

In your text, you should have a section of "Solubility Rules" that will let you decide whether any particular ionic compound is soluble or insoluble. You will probably need to learn those rules. I'm pretty sure that one of the rules has to do with transition metal halides being insoluble.

The equation that you wrote has a couple of errors in it. Make sure and check out the equation, too, so that you really do have the correct formulas for the compounds involved.