Question

during early research in nuclear energy, why were scientists surprised that the products from splitting a uranium nucleus had less mass than the reactants?
they expected that the large release of energy would cause an increase in mass.
up to that time, only chemical changes were known to form less mass than they started with.
all known changes always started and ended with the same amount of mass.
other known nuclear changes resulted in the same mass of products as reactants.

Explanation:

According to the law of conservation of mass (before Einstein's mass - energy equivalence was well - understood in nuclear reactions), in all known chemical and most physical changes at that time, the mass of reactants was equal to the mass of products. Scientists were operating under the assumption of strict mass conservation for all changes. When they saw mass loss in uranium fission (a nuclear change), it was a surprise because of this pre - existing belief in universal mass conservation for all changes.

Answer:

All known changes always started and ended with the same amount of mass.