Question

Why is mass in a nuclear reaction not conserved?

Answer 1

Some mass is turned into energy, according to `E=mc^2`.

Explanation:

`E=mc^2` is probably the most famous equation. `E` is the energy, `m` is mass, and `c` is the constant speed of light. Einstein came up with it to show that energy and mass are proportional - one can turn into the other, and back again.

Mass in nuclear reactions is not strictly conserved due to this principle of mass and energy being quite similar. We know that nuclear reactions release a lot of energy. This energy, though, is actually mass that is lost from nucleons, converted into energy, and lost as the mass defect.