Let's say for example I have 5g of copper in a beaker. Mass would be 5g + mass of beaker. If I add any given amount of water to the beaker containing the copper, that will change the mass of the beaker containing the copper. Am I correct so far? So then if I pour out the water, but keep all the solid copper inside the beaker, the copper is still wet. So I leave the beaker containing the copper to dry overnight, the water "vaporizes" and then do I now have the original mass of 5g of copper + mass of beaker?
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Yes as long as the copper you left wet doesn't oxidize or otherwise react with something OR if the wet stuff left is not pure H2O. Water from the faucet usually contains Ca and Mg salts and when the liquid evaporates it leaves a very small residue in a beaker. It isn't much but it's enough to weigh. But as long as everything is pure and no other reactions occur you are right.
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