Formulae, equations and conservation of mass
🎯What you need to be able to do
- Distinguish physical and chemical changes and recognise signs of a chemical reaction.
- Work out chemical formulae from ion charges (valencies), including compound ions.
- Write word equations and balanced symbol equations with state symbols.
- Explain the law of conservation of mass, including reactions that seem to gain or lose mass.
- Classify reactions as synthesis, decomposition, displacement, neutralization, combustion or precipitation.
💥Physical and chemical change
In a physical change (melting, dissolving, boiling) no new substance forms and the change is easily reversed. In a chemical change (a reaction) new substances form with different properties, and the change is usually hard to reverse. Signs of a reaction include a colour change, gas given off (fizzing), a precipitate (solid forming in a solution), a temperature change, or light and sound.
🔢Writing formulae
A formula shows the elements in a compound and the ratio of their atoms. For ionic compounds, the charges must add up to zero. Use the ion charges below, and bracket compound ions when you need more than one.
| Positive ions | Negative ions |
|---|---|
| +1: H+, Na+, K+, Ag+, NH4+ (ammonium) | −1: Cl−, Br−, OH− (hydroxide), NO3− (nitrate), HCO3− |
| +2: Mg2+, Ca2+, Cu2+, Zn2+, Fe2+ | −2: O2−, S2−, SO42− (sulfate), CO32− (carbonate) |
| +3: Al3+, Fe3+ | −3: N3−, PO43− (phosphate) |
Swap-and-drop: for aluminium oxide, Al3+ and O2−: swap the numbers to give Al2O3 (charges: 2 × +3 = +6, 3 × −2 = −6 ✓). Calcium hydroxide: Ca2+ and OH− give Ca(OH)2. Names ending in -ide have two elements; -ate means oxygen is included too.
⚖️Conservation of mass
In a chemical reaction atoms are rearranged, never created or destroyed. So the total mass of the products equals the total mass of the reactants. When a reaction seems to lose mass (marble chips fizzing in acid) a gas has escaped into the air; when it seems to gain mass (magnesium burning) it has combined with oxygen from the air. In a sealed container the mass stays exactly the same.
📝Balancing equations
A balanced symbol equation has the same number of each type of atom on both sides. You balance by changing the numbers in front of formulae (coefficients), never the small subscripts — that would change the substance. Add state symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous, dissolved in water.
✏️Worked example: balancing a combustion
1. Unbalanced: CH4 + O2 → CO2 + H2O
2. Carbon: 1 on each side ✓.
3. Hydrogen: 4 on the left, 2 on the right. Put 2 in front of H2O: CH4 + O2 → CO2 + 2H2O.
4. Oxygen: right side now has 2 + 2 = 4 O atoms, so we need 2O2 on the left.
📂Types of reaction
| Type | Pattern | Example |
|---|---|---|
| Synthesis | A + B → AB | 2Mg + O2 → 2MgO |
| Decomposition | AB → A + B | CaCO3 → CaO + CO2 (thermal decomposition) |
| Displacement | A + BC → AC + B | Zn + CuSO4 → ZnSO4 + Cu |
| Neutralization | acid + base → salt + water | HCl + NaOH → NaCl + H2O |
| Combustion | fuel + oxygen → oxides | CH4 + 2O2 → CO2 + 2H2O |
| Precipitation | two solutions → insoluble solid | AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq) |
Some reactions are reversible (⇌): the products can react to re-form the reactants. Heating blue hydrated copper(II) sulfate drives off water to leave white anhydrous copper(II) sulfate; adding water turns it blue again. See Topic 11.
🌎Science in context: fertilizers and finite resources
Balanced equations let industry calculate exactly how much raw material it needs — and how much waste it produces. Fertilizer plants in Indonesia turn natural gas into ammonia and urea for rice farming. Phosphate fertilizers depend on phosphate rock, a finite resource mined in a handful of countries. Imbalances in who owns and can afford such resources are a real Criterion D question about fairness and food security.
🧠Quick check
1. Write the formula of magnesium nitrate and of sodium sulfate.
Mg(NO3)2 and Na2SO4.
2. Balance: H2 + O2 → H2O
2H2 + O2 → 2H2O.
3. Balance: Al + O2 → Al2O3
4Al + 3O2 → 2Al2O3 (Al: 4 = 4; O: 6 = 6).
4. 12.0 g of magnesium burns to give 20.0 g of magnesium oxide. What mass of oxygen reacted?
20.0 − 12.0 = 8.0 g, by conservation of mass.
5. Why does the mass of an open flask of acid and marble chips decrease?
Carbon dioxide gas is produced and escapes into the air. The total mass (including the gas) is conserved.
6. What type of reaction is 2KClO3 → 2KCl + 3O2?
Decomposition (thermal decomposition when heated): one compound breaks down into simpler substances.
📝Worksheet
Test yourself on the whole topic with a printable worksheet: questions for all four criteria, from recall to a design task, a data-analysis question and a short reflection, with a full mark scheme.
Worksheets are for members — sign in or join. The topic 1 worksheet is a free sample.