If a reaction occurs in an open flask and the observed mass decreases, which explanation best fits with the Law of Conservation of Mass?
A gas has escaped from the flask
The Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction; total mass of products equals total mass of reactants.
In an open flask, a gaseous product (e.g. CO2 when marble reacts with acid) can escape into the air, so the mass left in the flask appears to decrease.
If the gas were collected (closed system), the total mass would be unchanged, so the law still holds.
Reactants do not become lighter, the law is not invalid, and mass–energy conversion in chemical reactions is far too small to be observed.
Hence, the answer is A gas has escaped from the flask.
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