This mini-lesson covers the whole of CCEA's Symbols, Formulae & Equations work: writing the formulae of elements and compounds, using the charges of common ions to balance charges in an ionic formula, then writing word equations and balanced symbol equations with state symbols, plus ionic equations (Higher) and conservation of mass.
Work through each screen, answer the questions as you go (some choose a formula, some give a balancing number) and collect ⭐ stars. Press Start when you're ready.
Each element has a chemical symbol — one or two letters, e.g. H for hydrogen, O for oxygen, Na for sodium. A chemical formula uses those symbols to show which atoms a substance contains and how many of each.
Common molecules to know: water H₂O, carbon dioxide CO₂, ammonia NH₃, methane CH₄, hydrogen chloride HCl. The subscripts come from how many bonds each atom forms — they are fixed, so never change them to balance an equation.
When metals and non-metals form compounds they make ions — atoms (or groups of atoms) with a charge. To write the formula of an ionic compound you first need the charges. Learn these:
Spot the pattern: for the simple ions, Group 1 → +1, Group 2 → +2, Group 3 → +3, Group 6 → −2 and Group 7 → −1. The charge (valency) is what you balance.
Pick the correct formula for each pair of ions — balance the charges so the compound is neutral.
An ionic compound is overall neutral: the total + charge must cancel the total − charge. Take the charge on each ion and swap it to become the number of the other ion — the criss-cross rule:
Watch out: if the charges are equal they just cancel 1:1 — Na⁺ + Cl⁻ → NaCl (not Na₁Cl₁), and Mg²⁺ + O²⁻ → MgO. If a polyatomic ion needs more than one, wrap it in brackets: Ca²⁺ + NO₃⁻ → Ca(NO₃)₂.
A word equation names the reactants (starting substances) on the left and the products (what is made) on the right, joined by an arrow → meaning "react to form":
The arrow is one way for most reactions. A + separates two reactants or two products. Word equations show what reacts; symbol equations (next) show how much.
Tip: "burns in" or "combustion in air" means reacting with oxygen; a metal + acid makes a salt + hydrogen; an acid + alkali makes a salt + water.
A symbol equation swaps the names for formulae. Because no atoms are made or lost, each element must have the same number of atoms on both sides. You balance by putting a big number (a coefficient) in front of a formula — never by changing a subscript.
1. Write the correct formulae (never alter them).
2. Count atoms of each element on both sides.
3. Add coefficients to even them up — leave H and O till last.
4. Re-count to check, then add state symbols.
A finished symbol equation should show the physical state of each substance with a small bracket after its formula:
Common slip: water made in a reaction is usually (l), but a substance dissolved in water (like an acid or a salt solution) is (aq), not (l).
In a chemical reaction no atoms are created or destroyed — they are only rearranged. So the total mass of the products equals the total mass of the reactants. This is the conservation of mass, and it is exactly why a symbol equation must balance.
"But it got lighter!" When magnesium burns it seems to vanish, and when a metal rusts it gets heavier — in both cases a gas (oxygen) joins or leaves. Count all reactants and products, gases included, and the mass always balances.
Tap an equation, then tap the box. Is it balanced (same atoms each side) or not?
When ionic substances react in solution, some ions take part and others just float by unchanged — the spectator ions. An ionic equation shows only the ions that actually react, leaving the spectators out.
For example, mixing silver nitrate with sodium chloride solution:
Two checks for an ionic equation: the atoms must balance and the total charge must be the same on both sides (here −1+1 = 0 on the left, and 0 on the right).
Formulae: read subscripts & brackets; know common molecules (H₂O, CO₂, NH₃, CH₄, HCl) and the diatomic gases.
Ions: recall the charges of common ions, including OH⁻, NO₃⁻, SO₄²⁻, CO₃²⁻, NH₄⁺.
Ionic formulae: balance charges (criss-cross) so the compound is neutral; bracket polyatomic ions.
Word equations: reactants → products.
Symbol equations: balance with coefficients (not subscripts) and add state symbols (s)(l)(g)(aq).
Ionic equations (Higher): cancel spectator ions; balance atoms and charge.
Conservation of mass: atoms are rearranged, never lost — mass is conserved.
You've covered the whole of CCEA's Symbols, Formulae & Equations. Press Finish to see your score.
You've worked through Formulae & Equations for CCEA GCSE Chemistry. 🎉
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