Quantitative chemistry · GCSE Chemistry
Conservation of mass
GCSE Chemistry revision on conservation of mass: atoms rearranged not created, closed versus open systems, and why a flask can lose mass when a gas escapes.
Atoms are rearranged, not created or destroyed. In a closed system the total mass stays the same. If a gas leaves an open flask, the flask’s reading falls — the atoms are still in the gas.
The important bits
What you need to know
- 1
The law of conservation of mass: the total mass of reactants equals the total mass of products in a closed system, because atoms are only rearranged.
- 2
A balanced symbol equation is conservation written as chemistry: 2Mg + O₂ → 2MgO has two Mg and two O on each side.
- 3
If magnesium burns in an open crucible, the solid product MgO has a greater mass than the original Mg because oxygen from the air has joined. The extra mass came from O₂(g).
- 4
If calcium carbonate is heated in an open tube, the mass of solid falls because CO₂(g) escapes: CaCO₃(s) → CaO(s) + CO₂(g). In a closed system the total mass would still be constant.
- 5
Precipitation: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq). Mixing two solutions, the total mass of the beaker contents stays the same; a white solid appears but no atoms vanish.
- 6
Relative formula mass lets you check an equation: Mᵣ(CaCO₃) = 100, Mᵣ(CaO) = 56, Mᵣ(CO₂) = 44, and 56 + 44 = 100.
- 7
State symbols explain the weighing: (g) products can leave; (s) and (l) usually stay. Cotton wool on a flask lets CO₂ out while you measure mass loss for a rate practical.
- 8
You cannot “lose mass” as energy in these school reactions in any way that changes atom counts. Energy transfer does not destroy atoms. Apparent mass change is always atoms arriving or leaving.
Quotations worth analysing
Short evidence. Real method.
“2Mg(s) + O₂(g) → 2MgO(s)”
The solid gets heavier because oxygen atoms have been added. If you could weigh the air that was used, total mass would still balance.
“CaCO₃(s) → CaO(s) + CO₂(g)”
The missing mass is carbon dioxide gas. Students say “mass is destroyed by heating”. Count the atoms: none have gone missing from the universe.
“Total mass of reactants = total mass of products (closed system).”
Closed means nothing can enter or leave. A sealed flask of reacting solutions keeps a constant mass even if a gas is produced inside.
Go deeper
Decide whether atoms are joining from the air or leaving as a gas
Two classic practicals look like they break conservation if you only watch the solid. Burning magnesium: lid on a crucible, lift slightly so oxygen can enter, weigh before and after. Mass of MgO > mass of Mg. Heating copper carbonate: green solid to black CuO, mass down, limewater cloudy from CO₂. In both cases write the equation with state symbols, then say where the extra or missing mass is. A question that gives masses of all substances including the gas should add up. If 10.0 g CaCO₃ makes 5.6 g CaO, the CO₂ is 4.4 g. That is the mole page and this page holding hands.
Go deeper
Closed versus open is the command-word trap
“Explain why the mass of the flask decreased” wants: a gas was produced and escaped. “Explain why mass is conserved” wants: atoms are rearranged, none created or destroyed. Both can be true of the same experiment, depending whether you weigh the flask or the whole system. In precipitation, students expect the mass to rise because a solid “appears”. The solid was ions in solution; they have only packed into a lattice. Total mass is unchanged. If the question shows a sealed container of hydrogen and oxygen exploded to make water, the mass of the container is constant even though gases became a liquid.
Go deeper
Balancing equations is conservation of atoms
You may not change formulae to make an equation balance. You may only change the big numbers. H₂ + O₂ → H₂O is not balanced; 2H₂ + O₂ → 2H₂O is. Count each element. Charges must also balance in ionic equations: Fe + Cu²⁺ → Fe²⁺ + Cu, not Fe + Cu²⁺ → Fe + Cu. A balanced equation is the only safe starting point for a mole calculation. If your masses from n × Mᵣ do not conserve when you include every substance, the equation or an Mᵣ is wrong. Use that as a check, not as an afterthought.
See the idea in action
A student heats 10.00 g of CaCO₃ in an open crucible until the mass of solid is constant at 5.60 g. Equation: CaCO₃ → CaO + CO₂. Mass of CO₂ escaped = 10.00 − 5.60 = 4.40 g. Check with Mᵣ: 10.00 g CaCO₃ is 0.100 mol, so 0.100 mol CO₂ is 4.40 g and 0.100 mol CaO is 5.60 g. If the same 10.00 g were heated in a sealed container of fixed volume, the total mass of container plus contents would stay 10.00 g plus the original air, because the CO₂ would still be inside.
Exam technique
Turn knowledge into marks
Name the gas that leaves or the oxygen that joins, then say atoms are conserved. Use a balanced equation and Mᵣ to predict the mass change. Do not write that heat destroys mass.
Common mistakes
Do not give these marks away
- 01
Saying mass is destroyed when a gas escapes, or created when magnesium burns.
- 02
Forgetting oxygen from the air in combustion mass-gain experiments.
- 03
Changing chemical formulae to “balance” an equation instead of changing the big numbers.
Why does the mass of a crucible plus contents increase when magnesium burns in air?
AMass is created by the flame
BOxygen from the air combines with magnesium to make MgO
CThe crucible absorbs carbon dioxide
DMagnesium atoms get heavier when they are heated
Show the answer
Oxygen from the air combines with magnesium to make MgO. 2Mg + O₂ → 2MgO. The extra mass is oxygen atoms that have joined. In a closed system that included the air used, total mass would be constant.
Quick questions
If this is the bit you searched
What is conservation of mass GCSE Chemistry?
In a closed system the total mass of reactants equals the total mass of products, because atoms are rearranged, not created or destroyed.
Why does mass decrease when calcium carbonate is heated?
Carbon dioxide gas is produced and escapes from an open container: CaCO₃ → CaO + CO₂. The atoms of CO₂ still exist; they have left the crucible.
Why does magnesium gain mass when it burns?
It combines with oxygen from the air to form MgO. The solid product includes those oxygen atoms, so it is heavier than the original metal.
Does a precipitation reaction change the total mass?
Not if you weigh all the solutions and the beaker. Ions that were dissolved form a solid, but no atoms leave or arrive.