Common Precipitate Lab Errors

Time to explain a few things a bit more. 
I hope you read this before the midterm!

Ionic compounds have cations and anions bonded together with ionic bonds.  That means electrons are transferred from the metals (which become positive cations) to the nonmetals (which become negative) and then they are wildly attracted.

When you put an ionic compound into water, most of them dissolve away invisibly.  When that happens, the compound DISSOCIATES or IONIZES, into loose cations and anions that float around in the water.  Since they are matched up equally, the solution remains neutral (the positives equal the negatives).  But, since there are loose, mobile ions, this solution can conduct electricity, and is therefore an electrolyte.

When you put a molecular compound into water, it might dissolve too.  But if it does, it does so in a TOTALLY DIFFERENT way than an ionic compound would.  First of all, there are no metals or ions in a molecular compound.  The atoms are bonded by SHARING electrons, not by transfer.  Neutral atoms bond with other neutral atoms, making neutral compounds.

If the molecular compound dissolves into water, the molecules come apart from each other, forming loose molecules.  Since there are NO loose or mobile ions (no ions at all!) this molecular solution can’t conduct electricity and is not an electrolyte.

If you put something like calcium sulfate into water, it is ionic (formed from metal cations and nonmetal anions).  Since is DOES NOT DISSOLVE, it’s not aqueous, there are no loose, mobile ions in solution, this is not going to conduct electricity, it’s not an electrolyte.

NaCl(S)    Na+1(AQ) + Cl-1(AQ)
These are LOOSE and MOBILE IONS in the solution.

C12H22O11(S)  →  C12H22O11(AQ)   

These are loose (and mobile) molecules – NOT IONS.  There are no ions in a molecular compound.  Since there are no ions, this is not going to conduct electricity.  This is not an electrolyte either.

CaSO4 goes into water to form nothing, the calcium sulfate is not aqueous in water.  There are no ions present, and none in the water.
Although the CaSO4 is ionic, it’s not an electrolyte.
Although the diagram below is about AgCl, not CaSO4, both are insoluble in water, they are similar enough for you to “get” this picture.
When calcium sulfate is added to water, it remains as solid CaSO4 – it does not dissolve into independent ions.