Conservation of mass and balanced equations
In any chemical reaction, atoms are rearranged, not created or destroyed. So the total mass of reactants always equals the total mass of products — the law of conservation of mass.
What balanced equations show
A balanced equation shows the same number of each type of atom on both sides. For example:
2H₂ + O₂ → 2H₂O
- Reactants: 4 H + 2 O
- Products: 4 H + 2 O ✓
Without balancing numbers, the equation would say "2 hydrogens turn into 2 hydrogens and 2 oxygens" — impossible.
How to balance an equation step-by-step
- Write the unbalanced word and symbol equation.
- Count atoms on each side.
- Add large numbers (coefficients) in front of formulae — never change subscripts inside formulae.
- Tackle the most complex molecule first, leave H and O until last.
- Recount; check.
Worked example: balance CH₄ + O₂ → CO₂ + H₂O
- Carbon: 1 = 1 ✓
- Hydrogen: 4 ≠ 2 → need 2H₂O
- Oxygen: 2 ≠ (2 + 2 = 4) → need 2O₂
- Final: CH₄ + 2O₂ → CO₂ + 2H₂O
Double-check: C: 1=1; H: 4=4; O: 4=4. ✓
Apparent mass changes — the two trap cases
Sometimes a reaction looks like mass changes. Both cases involve gases entering or leaving the system.
Case 1 — open vessel where mass appears to increase
Example: magnesium burning in air. 2Mg(s) + O₂(g) → 2MgO(s)
Oxygen from the air joins with magnesium; the solid product is heavier than the original metal. Mass is still conserved if you include the oxygen taken from the air.
Case 2 — open vessel where mass appears to decrease
Example: heating copper carbonate. CuCO₃(s) → CuO(s) + CO₂(g)
The CO₂ escapes as a gas; only the CuO is left in the crucible. Mass appears to decrease because the gas leaves.
In a sealed container, the total mass would not change.
State symbols
Always include where useful:
- (s) solid
- (l) liquid
- (g) gas
- (aq) aqueous (dissolved in water)
Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
⚠Common mistakes
- Changing subscripts inside formulae. Don't write "H₂O₂" to balance H₂O — that's hydrogen peroxide. Only adjust the coefficients in front.
- Forgetting to count atoms across multiple molecules. "2H₂O" has 4 H atoms.
- Not balancing diatomic elements. O₂, N₂, H₂, Cl₂, F₂, Br₂, I₂ are diatomic — count the molecule's atoms.
- Saying "the mass changes when iron rusts". It only appears to change because oxygen joins from the air; total mass is conserved.
✦Worked example— Worked example with state symbols
Balanced equation for sodium reacting with water: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
Check: Na: 2=2; O: 2=2; H: 4=4 ✓
Links
Foundation for C3.2 (Mr) and C3.4–3.6 (moles, masses). Conservation of mass is examined repeatedly across C4 and C9.
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