For every chemical change you will meet, the answer is no — mass is conserved, provided the system is closed. But the honest, deeper answer has a small and beautiful exception.
1. The apparent exceptions are not exceptions at all. Every case where a balance seems to disagree turns out to be an open system:
- Gas escapes — burning ethanol in an open beaker, or vinegar and baking soda in an unstoppered flask. The reading falls.
- Gas is absorbed from the air — magnesium burning, or iron rusting. Here the reading actually rises, because oxygen from the air joins the sample. Seal the system and both effects vanish.
2. The real, tiny exception. Einstein showed that energy and mass are two forms of the same thing, linked by E = mc². So a reaction that releases energy must also lose a corresponding amount of mass. Put a number on it for a typical reaction releasing about 400 kJ:
= (4 × 10⁵ J) ÷ (3 × 10⁸ m s⁻¹)²
= (4 × 10⁵) ÷ (9 × 10¹⁶) kg
≈ 4.4 × 10⁻¹² kg = about 4 nanograms
Four nanograms is a million times smaller than the 0.01 g that a good school balance can detect. So the loss is real but utterly unmeasurable, and for all chemical purposes the Law of Conservation of Mass holds exactly.
3. Where the loss is measurable. In a nuclear reaction the energy released is millions of times larger, and the mass loss becomes plain. When a uranium nucleus splits, the products weigh measurably less than the original nucleus, and that missing mass is what appears as energy in a nuclear power plant — the technology described in the Bridging Science and Society box on page 171. But note carefully: a nuclear change is not a chemical change. Chemistry rearranges the electrons around nuclei; it never touches the nuclei themselves.