A student says: 'Nuclear fusion releases energy because the total mass of the products is less than the total mass of the reactants.' Explain why the products having less mass means that energy is released, and describe the conditions needed for nuclear fusion to occur.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The missing mass has been converted into energy (mass‑energy equivalence, Einstein’s equation E=mc²). Because c (the speed of light) is very large, even a tiny mass loss produces a very large amount of energy. Nuclear fusion requires extremely high temperatures (millions of degrees, comparable to the Sun’s core). The nuclei must be moving fast enough to overcome the electrostatic repulsion and get close enough to fuse. Very high pressure or density is also needed to increase the collision frequency and confine the plasma.
Examiner tips
- Use the phrase ‘mass‑energy equivalence’ and write E=mc² explicitly.
- Mention the huge value of c to justify large energy release.
- State both high temperature and high pressure/density as required conditions.
- Keep the answer concise – 5 marks, so one sentence per point.
Common mistakes
- Confusing mass loss with energy loss, or saying energy is ‘created’ rather than ‘converted’ from mass.
- Omitting the need for high pressure/density or the electrostatic repulsion barrier.
- Using informal language such as ‘big energy’ instead of ‘large amount of energy’.
Mark scheme (5 marks)
- The missing mass has been converted into energy (mass-energy equivalence / Einstein's equation E = mc²)
- Because c (the speed of light) is very large, even a tiny mass produces a very large amount of energy
- Nuclear fusion requires extremely high temperatures (accept: millions of degrees / temperatures comparable to the core of the Sun)
- The nuclei must be moving fast enough (have enough kinetic energy) to overcome the electrostatic repulsion between them / get close enough to fuse
- Very high pressure / density is also needed to increase the chance of nuclei colliding (accept: to confine the plasma / to increase collision frequency)
Key terms in this question
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