Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- A chemical equation records atoms being rearranged. It must conserve atoms (so mass) and charge.
- Balance with coefficients, never by changing subscripts.
- Physical changes have equations too: only the state symbol changes, or an ionic solid separates into its ions, e.g. CaCl₂(s) → Ca²⁺(aq) + 2Cl⁻(aq).
- One reaction in solution can be written three ways: molecular, complete ionic and net ionic.
Key relationships
| Form | What it shows | Use it to |
|---|---|---|
| Molecular | every substance as a full formula | plan amounts of reagents |
| Complete ionic | strong electrolytes as separate ions | list every species; find spectators |
| Net ionic | only the species that change | show what actually reacts |
| Split into ions | Keep whole |
|---|---|
| soluble ionic compounds (aq) | solids and precipitates (s) |
| strong acids: HCl, HBr, HI, HNO₃, HClO₄, H₂SO₄ | liquids, including H₂O(l) |
| soluble strong bases (NaOH, KOH) | gases (g) |
| weak acids and molecular substances |
All sodium, potassium, ammonium and nitrate salts are soluble. Other solubility data will be given, or shown by a precipitate.
Assumptions behind the rules
- “Strong” acids and soluble ionic compounds are treated as fully separated into ions in water.
- A precipitate is treated as completely insoluble, so it is written as a solid.
- H⁺(aq) and H₃O⁺(aq) mean the same thing here; H₃O⁺ is preferred.
Mistakes to avoid
- Splitting a solid or a gas into ions.
- Breaking polyatomic ions such as SO₄²⁻ into atoms.
- Losing coefficients when splitting: 3Ca(NO₃)₂ → 3Ca²⁺ + 6NO₃⁻.
- Checking atoms only. Add up the charges too.
- Calling an ion a spectator when it changes state (Ca²⁺ from solid CaCO₃ is not a spectator).
Quick self-check
- Is Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) balanced for charge? (Yes: +2 on each side.)
- Write the equation for solid AlCl₃ dissolving in water. (AlCl₃(s) → Al³⁺(aq) + 3Cl⁻(aq))
- Mg(s) is added to HCl(aq), giving MgCl₂(aq) and H₂(g). Net ionic equation? (Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g); Cl⁻ is the spectator.)
Next: practice questions.