|MadSci Network: Chemistry|
There are a few possible reasons for this. One of them is *NOT* that the mass of the reaction is not conserved! 1) The plastic bag may not be perfectly air-tight, and some of your gas may be escaping. 2) Whereas carbon dioxide is indeed heavier than air, it is not so heavy that all of the molecules sit down on the surface of the liquid. The gas is "floating", and thus is exerting a partial pressure on the inside of the plastic bag. In other words, it's pushing up a little bit, and reducing the normal downward push of atmospheric pressure. Because of this, using reactions that generate gas may not be the best way to demonstrate conservation of mass. I recommend using a solution of copper sulfate in one small flask and a solution of barium chloride in another. Place both on a balance and check the mass, and then pour the two together. Replace both flasks on the balance and record the mass once more. The reaction forms a nice heavy precipitate.
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