I recently made a saturated copper sulfate solution, and let it react with iron to create ironsulfate and copper. As expected it created copper on the bottom of the glass, and the solution turned from a dark blue to light green, usually indicating ironsulfate. I was a little confused when the reaction started releasing small amounts of gas. After filtering the copper out of the solution and letting is stay over night, I discovered a yellow residue floating close to the bottom of the glass. I did some research, but didn't find anything that could help me. Does anyone know what that residue is and what I might have done wrong?