If the total mass of the products of a reaction is less than the mass of the reactants, what must be true?
A nuclear reaction has taken place.
A chemical reaction has taken place.
Nuclear fusion has occurred.
Nuclear fission has occurred.
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Because of energy conservation, the mass cannot "disappear", it just changes form.
A nuclear reaction can either be a nuclear fission or a nuclear fusion
Nuclear fission is the splitting of a large atom into two or more smaller ones.
Fusion is the fusing of two or more lighter atoms into a larger one.
In a chemical reaction, reactants are changed into products. Mass is conserved, but the reaction can yield gas so that the products appear to weight less than the reactants.
During both fusion and fission, mass appears to decrease due to the so called mass decay, where some particles are converted to energy.
I would guess that your answer is " A nuclear reaction has taken place ".