Topic 2 of 6
Balance atoms, then account for ions
A reaction changes arrangement, not the total number of atoms.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Balance atoms, then account for ions
A reaction changes arrangement, not the total number of atoms.
Write correct formulae first, then change coefficients until each element has the same atom count on both sides. Never alter a subscript to balance an equation. State symbols distinguish solid (s), liquid (l), gas (g) and aqueous solution (aq). Aqueous means dissolved in water, not simply liquid.
Worked example
Balance combustion
Write the complete combustion of propane.
- Start C3H8(g) + O2(g) -> CO2(g) + H2O(l).
- Balance carbon: 3CO2. Balance hydrogen: 4H2O.
- Products contain ten O atoms, requiring 5O2.
C3H8(g) + 5O2(g) -> 3CO2(g) + 4H2O(l). Water is shown after cooling; in hot exhaust it is vapour.
An ionic equation removes spectator ions that remain unchanged in solution. Split dissolved ionic compounds into ions; keep solids, gases, water and undissociated molecular substances intact. Cancel identical ions on both sides and check both atoms and total charge.
Worked example
Find the reacting ions
Aqueous silver nitrate reacts with aqueous sodium chloride.
- AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq).
- Dissolved reactants provide Ag+, NO3-, Na+ and Cl-.
- Na+ and NO3- remain dissolved: cancel them.
Ag+(aq) + Cl-(aq) -> AgCl(s). Total charge is zero on both sides.