Conservation reasoning is the idea that a reaction rearranges atoms and never creates or destroys them, so atoms, mass and charge must balance. Every mole calculation in SPM Chemistry depends on it.
This cluster sits under moles, formulas and equations. If you need the routine first, see balancing chemical equations.
What do the four pages cover?
- Connecting particle ratios to mole ratios without treating coefficients as masses: why the coefficients count particles, not grams.
- Identifying the limiting reactant from a complete original dataset: which reactant runs out first.
- Explaining a smaller measured yield without changing the balanced equation: why less product is collected than predicted.
- Checking mass and charge independently in an ionic equation: two separate checks that both must pass.
A practice page sits at the end.
One reaction, three checks
Take magnesium reacting with hydrochloric acid: Mg + 2HCl → MgCl₂ + H₂. Atoms: one Mg, two H and two Cl on each side. Mass: 24 + 2 × 36.5 = 97 on the left, and 95 + 2 = 97 on the right.
Charge is zero on both sides in the full equation. In the ionic version, Mg + 2H⁺ → Mg²⁺ + H₂, the left charge is +2 and the right charge is +2. One idea, three checks, and the reaction passes all of them.
Who should start where?
A student who can follow a mole calculation but cannot explain it should begin at the first page. A student preparing for questions that say “explain” or “justify” can go straight to the yield page.
Use the mole and stoichiometry steps tool to rehearse the routine, and redox and equilibrium to see charge conservation in electron transfer. For a teacher to question your reasoning, see online one-to-one Chemistry tuition.