Hydrogen and oxygen react explosively producing water, according to the reation:

True (Green) or False(Pink): the mass of two H₂O molecules is exactly equal to the mass of 2 hydrogen molecules plus the mass of one oxygen molecule.

Answer: False. Energy is mass, mass is energy. This is true for all reactions, not just nuclear reactions. If energy is produced in the reaction, some mass was converted into energy and the mass of the products is slightly less than the mass of the reactants. In a chemical reaction, the mass loss is too small to measure.