<span>If you don't properly clean a crucible before weighing it, the mass that you record will be greater than the actual mass of the crucible. If that extra weight is lost during the course of an experiment and you reweigh the full crucible later, you will not account for some of the product generated; that is, your final mass of the full crucible minus the original mass of the crucible will generate a value lesser than the one that you SHOULD.
So, for this experiment, I gather that you placed in a crucible some sort of hydrated salt and then heated it to remove the water. If, in addition to removing water, you also removed some sort of impurity from the crucible, it is going to appear that you have lost more water weight than you actually have. So, you will be reporting a mass percent of water that is too high.</span>
The reported mass will be too high because the initially recorded mass will include both water and impurities and the final mass will without both components. The difference will be larger so the amount of water reported would be higher.
