Question

In nuclear fission, which of the following quantities is conserved?
A. Energy
B. Mass
C. Momentum
D. Total energy and mass

Answer 1

Hint: In nuclear fission, heavy nucleus (unstable) breaks into two or more small product nuclei. When we compare the masses of reactants and products we will find a slight difference between the two known as mass defects.

Complete step by step answer:
For the sake of understanding better, we consider the following nuclear reaction:
$^{235}_{92} U + n \rightarrow ^{144}_{56} Ba + ^{89}_{36} Kr + 3n + 200 MeV$
Here, when a neutron (thermal/slow) is incident on Uranium nucleus, it Breaks down into Krypton and Barium and also gives 3 neutrons. When we compare the mass numbers on both sides we will find that the total mass number of the products is equal to that of reactants. But, mass number is not a precise measure of mass. It does not take into account the Binding energy. Therefore, when we write experimentally determined masses on both sides of the equation, we will find a mass difference of about 200 MeV, which is released energy.
Therefore what is mass on one side, appears as energy on the other side. This result also follows from the famous mass-energy equivalence relation given by Einstein:
$E = mc^2$.
Therefore in a nuclear reaction we consider conservation of total mass along with total energy. Conservation of either one of them cannot be considered individually else we might witness sudden production of energy out of nowhere in the product side.
Besides this, conservation of momentum is also followed in nuclear reactions and is often used in the analysis of problems.

Therefore, the correct answers are (C) and (D).

Note:
Any amount of mass can be written in terms of energy if we simply multiply it by square of speed of light. This is why we write the mass defect to be 200 MeV which is equivalent to about 0.215 u (unified mass).