Balance any chemical equation instantly, identify reaction types, generate ionic equations, and practise with 50+ equations. Step-by-step solutions with atom count verification for GCSE and A-Level Chemistry.
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A balanced chemical equation has the same number of atoms of each element on both sides of the arrow. This is a direct consequence of the Law of Conservation of Mass — atoms cannot be created or destroyed in a chemical reaction, only rearranged.
For example, the unbalanced equation Fe + O₂ → Fe₂O₃ has 1 iron atom on the left but 2 on the right, and 2 oxygen atoms on the left but 3 on the right. The balanced version is 4Fe + 3O₂ → 2Fe₂O₃, where both sides have 4 iron atoms and 6 oxygen atoms.
Balanced equations are essential for stoichiometry — calculating how much of each substance is needed or produced. The coefficients give the mole ratio, which is the foundation of all quantitative chemistry calculations. Without a balanced equation, you cannot accurately determine reacting masses, volumes, or concentrations.
The most common method taught at GCSE is balancing by inspection. Follow these five steps for any equation:
Chemical reactions are classified into 8 main types based on their patterns. Recognising these types helps you predict products and understand reaction mechanisms.
Two or more substances combine to form a single product
A single compound breaks down into two or more simpler substances
A more reactive element displaces a less reactive one from a compound
Two compounds swap partners to form two new compounds
A substance reacts with oxygen, releasing energy as heat and light
An acid reacts with a base to produce a salt and water
Involves transfer of electrons — one species is oxidised, another is reduced
Reactions that don't fit neatly into the above categories
Balancing by inspection means looking at the equation and adjusting coefficients one element at a time. This is the standard GCSE method and works well for simple to moderately complex equations.
Top tip: Start with the element that appears in the fewest compounds — this gives you fewer coefficients to adjust. Leave oxygen and hydrogen until last, as they often appear in multiple compounds.
For complex equations where inspection is difficult, the algebraic method assigns variables to each coefficient and creates simultaneous equations from atom counts. This is the method our calculator uses (via matrix algebra) and it works for any equation.
Matrix null-space method: Our calculator extends this algebraic approach using Gaussian elimination on a composition matrix. This automatically solves for all coefficients simultaneously, handling any equation — including ones with multiple valid solutions — without trial and error.
Oxidation numbers (oxidation states) track how many electrons an atom has effectively gained or lost. They are essential for identifying redox reactions — reactions where electron transfer occurs.
OIL RIG Mnemonic: Oxidation Is Loss of electrons (oxidation number increases). Reduction Is Gain of electrons (oxidation number decreases). In every redox reaction, one species is oxidised and another is reduced — electrons are transferred, not created or destroyed.
An ionic equation shows the individual ions present in a reaction. Soluble ionic compounds in aqueous solution are written as their separate ions, while insoluble solids, liquids, gases, and molecular compounds stay as complete formulae.
The net ionic equation removes spectator ions — ions that appear identically on both sides and don't participate in the reaction. This reveals the essential chemistry happening.
These 15 equations appear frequently in GCSE Chemistry exams. Make sure you can balance and recognise each one.
| Equation | Type |
|---|---|
| 2H₂ + O₂ → 2H₂O | Synthesis |
| 2Mg + O₂ → 2MgO | Synthesis |
| CH₄ + 2O₂ → CO₂ + 2H₂O | Combustion |
| CaCO₃ → CaO + CO₂ | Decomposition |
| 2H₂O₂ → 2H₂O + O₂ | Decomposition |
| Zn + CuSO₄ → ZnSO₄ + Cu | Displacement |
| Mg + 2HCl → MgCl₂ + H₂ | Displacement |
| HCl + NaOH → NaCl + H₂O | Neutralisation |
| H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O | Neutralisation |
| N₂ + 3H₂ → 2NH₃ | Synthesis |
| 2Fe₂O₃ + 3C → 4Fe + 3CO₂ | Redox |
| Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂ | Neutralisation |
| C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O | Combustion |
| 2Na + 2H₂O → 2NaOH + H₂ | Displacement |
| Fe + CuSO₄ → FeSO₄ + Cu | Displacement |
A-Level Chemistry introduces more complex equations including organic reactions, redox titrations, and equilibrium reactions.
| Equation | Type |
|---|---|
| 2KMnO₄ + 16HCl → 2KCl + 2MnCl₂ + 5Cl₂ + 8H₂O | Redox |
| C₈H₁₈ + 12.5O₂ → 8CO₂ + 9H₂O | Combustion |
| Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O | Redox |
| MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O | Redox |
| CH₃COOH ⇌ CH₃COO⁻ + H⁺ | Equilibrium |
| NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ | Equilibrium |
| C₂H₄ + H₂ → C₂H₆ | Addition |
| C₂H₄ + Br₂ → C₂H₄Br₂ | Addition |
| C₂H₅OH + CH₃COOH ⇌ CH₃COOC₂H₅ + H₂O | Esterification |
| 2SO₂ + O₂ ⇌ 2SO₃ | Equilibrium |
| 4NH₃ + 5O₂ → 4NO + 6H₂O | Redox |
| BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl | Precipitation |
| Fe²⁺ + 2OH⁻ → Fe(OH)₂ | Precipitation |
| Zn + 2H⁺ → Zn²⁺ + H₂ | Redox |
| Cu²⁺ + 4NH₃ → [Cu(NH₃)₄]²⁺ | Complex |
Balancing means adjusting coefficients so the same number of atoms of each element appears on both sides, satisfying the Law of Conservation of Mass.
Subscripts define the chemical formula of a substance. Changing H₂O to H₂O₂ changes water into hydrogen peroxide — a completely different compound. Only coefficients (the numbers in front) can be changed.
A coefficient is the number in front of a chemical formula (e.g. the 2 in 2H₂O). It means 2 molecules of water. It multiplies every atom in the formula.
State symbols show the physical state: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water. They're required in ionic equations and most A-Level equations.
The 8 main types: synthesis, decomposition, single displacement, double displacement, combustion, neutralisation, redox, and other. Each has a characteristic pattern of reactants and products.
A net ionic equation shows only the ions that participate in the reaction, removing spectator ions. For example, Ag⁺(aq) + Cl⁻(aq) → AgCl(s) is the net ionic for silver chloride precipitation.
Spectator ions appear in the same form on both sides of an ionic equation. They don't participate in the reaction and are removed to get the net ionic equation.
Assign oxidation numbers to each element in reactants and products. If any element's oxidation number changes, it's a redox reaction. Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain.
The mole ratio comes directly from the coefficients. In 2H₂ + O₂ → 2H₂O, the ratio is 2:1:2, meaning 2 moles of H₂ react with 1 mole of O₂ to make 2 moles of H₂O.
A balanced equation always has a unique set of smallest whole-number coefficients. You might see multiples (e.g. 4:2:4 instead of 2:1:2), but the simplest ratio is always unique.
Assign variables as coefficients, create simultaneous equations from atom counts, solve the system. This method works for any equation and is the basis of our matrix algebra approach.
If an element appears on one side but not the other, or if the products are wrong for the given reactants, no valid coefficients exist. Check your chemical formulae are correct.
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