Given below are two statements:
Statement I: In a nuclear reaction, mass and energy are conserved separately.
Statement II: The energy released in an exothermic chemical reaction also comes from a small decrease in the total mass of the reacting atoms and molecules.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Statement I is incorrect but Statement II is correct.
Statement I In a nuclear reaction, mass and energy are conserved separately. — False. By Einstein’s relation \( E=mc^{2} \), mass is another form of energy. In a nuclear reaction part of the rest mass changes into kinetic energy of the products, or the other way round. So neither mass nor energy is conserved on its own. What stays constant is the total of mass and energy taken together.
Statement II The energy released in an exothermic chemical reaction also comes from a small decrease in the total mass of the reacting atoms and molecules. — True. Chemical binding energy also gives a small negative contribution (a mass defect) to the mass of an atom or molecule. So the energy given out in a chemical reaction matches a decrease in total mass. But this mass difference is almost a million times smaller than in a nuclear reaction. It is too small to be noticed in practice.
NCERT (2026-27):
Points to Ponder – “3. After Einstein showed the equivalence of mass and energy, E = mc2, we cannot any longer speak of separate laws of conservation of mass and conservation of energy, but we have to speak of a unified law of conservation of mass and energy. …”
Example 13.4 – “(c) From the point of view of mass-energy interconversion, a chemical reaction is similar to a nuclear reaction in principle. The energy released or absorbed in a chemical reaction can be traced to the difference in chemical (not nuclear) binding energies of atoms and molecules on the two sides of a reaction. Since, strictly speaking, chemical binding energy also gives a negative contribution (mass defect) to the total mass of an atom or molecule, we can equally well say that the difference in the total mass of atoms or molecules, on the two sides of the chemical reaction gets converted into energy or vice-versa. However, the mass defects involved in a chemical reaction are almost a million times smaller than those in a nuclear reaction. …”