Watch a chemical reaction happen — a colour change, bubbles forming, heat suddenly released — and it looks like transformation, almost like something new has been created out of nothing. Zoom down to the level of individual atoms, and nothing nearly that dramatic is actually happening: existing bonds break, atoms rearrange, and new bonds form. Every atom that started the reaction is still there at the end, just organised differently.
GCSE coreConservation of mass: the rule everything else follows
In any chemical reaction, atoms are neither created nor destroyed — only rearranged into new combinations. This means the total mass of the reactants going in always exactly equals the total mass of the products coming out, even when it doesn't look that way (a burning log seems to vanish into ash, but most of its mass has simply become invisible gas, still made of the same atoms, dispersing into the air). This single rule is why chemical equations have to be balanced — the same number of each atom must appear on both sides, because nothing was ever added or lost.
A reaction doesn't create or destroy atoms — it breaks existing bonds and forms new ones, rearranging the same atoms into a different molecule with exactly the same total mass.
Exam-style question
4.6 g of sodium reacts completely with chlorine gas to form 11.7 g of sodium chloride. Calculate the mass of chlorine gas that reacted, explaining your reasoning. [3 marks]
Show model answer and mark breakdown
- By conservation of mass, the total mass of reactants must equal the total mass of products (1 mark)
- Mass of chlorine = mass of sodium chloride produced − mass of sodium used = 11.7 − 4.6 (1 mark)
- = 7.1 g (1 mark)
Examiner focus: state the conservation-of-mass principle explicitly as the reason for subtracting, not just perform the arithmetic — marks are usually available for the reasoning, not only the final number.
Common mistake: adding the two given masses instead of subtracting, which would incorrectly treat the sodium chloride as an extra product on top of unreacted sodium rather than as the reaction's only product.
Where the energy actually comes from
Breaking a chemical bond always requires energy input; forming a new bond always releases energy. Whether a reaction feels hot or cold to the touch comes down to which side of that balance wins. If forming the new bonds in the products releases more energy than it took to break the reactants' bonds in the first place, the surplus escapes as heat — an exothermic reaction, like combustion. If breaking the original bonds costs more energy than the new bonds give back, the reaction has to pull that extra energy in from its surroundings, which is why some reaction mixtures actually get colder as they react.
Why this makes reactions predictable, not mysterious
Once a reaction is understood as bond-breaking and bond-reforming rather than magical transformation, two things that look unrelated at GCSE — balancing equations and working out whether a reaction is exothermic or endothermic — turn out to be the same underlying idea viewed from two angles: atoms are conserved, and so, in a stricter sense, is energy. If chemical reactions, balancing equations or energy changes need explaining as one connected mechanism rather than separate topics to memorise, that's exactly what our GCSE chemistry tutoring is for — see the full learning pathway here.
Frequently asked questions
If mass is conserved, why does a burning log seem to lose so much mass, leaving only ash?
Because most of the products escape as gas — mainly carbon dioxide and water vapour — which you can't see or weigh once they've dispersed into the air. If you could capture and weigh every gas produced along with the ash, the total mass would exactly match the mass of the log plus the oxygen it reacted with. Nothing is destroyed, it's just no longer sitting in front of you as a solid.
What actually decides whether a reaction releases or absorbs energy?
It comes down to comparing the energy needed to break the reactants' bonds against the energy released when the products' new bonds form. If more energy is released forming new bonds than was used breaking old ones, the reaction is exothermic (releases energy, often as heat); if breaking the old bonds costs more than the new bonds release, it's endothermic (absorbs energy, often cooling its surroundings).
Why do chemical equations need to be balanced?
Because conservation of mass means the same number of each type of atom must appear on both sides of the equation — atoms aren't created or destroyed in the reaction, only rearranged into new molecules. A balanced equation is just the numerical bookkeeping that proves no atoms went missing or appeared from nowhere.
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Sudershan Soni
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