The law of conservation of mass states: in a chemical reaction, the total mass of the products equals the total mass of the reactants. No atoms are created or destroyed — they are simply rearranged.
This follows directly from Dalton's atomic theory: atoms are indivisible and indestructible. When copper reacts with sulfur to make copper sulfide, every single copper atom and every single sulfur atom ends up in the product — none disappear. The number of atoms before and after is identical, so the mass must be identical.
Apparent Mass Changes — Open vs Closed Systems
Students are often confused when a reaction appears to change mass. The key is whether the system is open or closed:
- Closed system: Nothing can enter or leave. Mass stays constant. If you burn magnesium ribbon in a sealed flask, the total mass does not change.
- Open system: Gases can escape or enter. Mass appears to change.
Why mass appears to decrease (open system): When calcium carbonate decomposes on an open balance — CaCO₃(s) → CaO(s) + CO₂(g) — the CO₂ gas escapes into the air. The balance only reads the solid mass remaining, so it appears lighter. In reality, if you collected all the CO₂ the total mass would equal the original CaCO₃.
Why mass appears to increase (open system): When magnesium burns in air — 2Mg(s) + O₂(g) → 2MgO(s) — the magnesium gains mass because it reacts with oxygen from the air. The oxygen atoms are added to the solid product, so the ash weighs more than the original ribbon.
Quick Check: 10 g of calcium carbonate is heated on an open balance. It produces 5.6 g of calcium oxide. What mass of gas escaped?
4.4 g of CO₂ escaped (10 − 5.6 = 4.4 g). Conservation of mass: total reactant mass = total product mass, so the CO₂ must account for the "missing" 4.4 g.
This how it works covers Why Mass Is Always Conserved within Formulae, Equations and Conservation of Mass for GCSE Chemistry. Revise Formulae, Equations and Conservation of Mass in Quantitative Chemistry for GCSE Chemistry with 13 exam-style questions and 12 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 6 of 14 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Formulae, Equations and Conservation of Mass
Which of the following is the correct formula for oxygen gas?
Explain why mass is conserved in a chemical reaction.