JEE Main · 2025 · 03 Apr 2025 · Shift 1 · Chapter_pending

In the first reaction a neutron splits ${}^{235}$U into ${}^{140}$Xe and ${}^{94}$Sr with two neutrons, which is nuclear fission. The second is the burning of hydrogen in oxygen, an ordinary chemical reaction. The third fuses two deuterons into ${}^3$He and a neutron; the products are lighter than the reactants, so energy is released and the $Q$ value is positive. In the fourth, ${}^1$H and ${}^3$H combine to give two deuterons whose combined mass exceeds that of the reactants, so the reaction absorbs energy and its $Q$ value is negative. Thus the uranium reaction is fission, the hydrogen-oxygen reaction is chemical, the deuteron-deuteron reaction is fusion with positive $Q$, and the proton-triton reaction is fusion with negative $Q$.