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There are millions of known chemical reactions, but a surprisingly small number of patterns covers most of the ones you’ll meet in an introductory course. Learning to recognize the pattern helps you predict products — which is often the hardest part of writing an equation.
1. Synthesis (combination)
Two or more substances combine to make one product.
A + B → AB
- 2H₂ + O₂ → 2H₂O
- 2Na + Cl₂ → 2NaCl
- CaO + H₂O → Ca(OH)₂ (slaking lime)
- N₂ + 3H₂ → 2NH₃ (the Haber process)
How to spot it: several reactants, one product.
2. Decomposition
One substance breaks down into two or more simpler ones. The reverse of synthesis. Usually needs energy — heat, light or electricity.
AB → A + B
- 2H₂O → 2H₂ + O₂ (electrolysis)
- CaCO₃ → CaO + CO₂ (heating limestone)
- 2H₂O₂ → 2H₂O + O₂ (hydrogen peroxide decomposing, sped up by a catalyst)
- 2KClO₃ → 2KCl + 3O₂ (a classic lab source of oxygen)
How to spot it: one reactant, several products.
3. Single displacement (single replacement)
A more reactive element pushes a less reactive one out of a compound.
A + BC → AC + B
- Zn + CuSO₄ → ZnSO₄ + Cu (zinc coats with copper; the blue solution fades)
- Fe + CuSO₄ → FeSO₄ + Cu
- Mg + 2HCl → MgCl₂ + H₂ (metal + acid, fizzing hydrogen)
- Cl₂ + 2KBr → 2KCl + Br₂ (a more reactive halogen displaces a less reactive one)
Whether it happens depends on the reactivity series: a metal can only displace one below it. Copper won’t displace zinc, so Cu + ZnSO₄ → no reaction. See the most reactive metals.
How to spot it: an element + a compound → a different element + a different compound.
4. Double displacement (double replacement, metathesis)
Two compounds swap partners. Usually in solution, and it only “goes” if something drives it: a precipitate, a gas, or water forms.
AB + CD → AD + CB
- AgNO₃ + NaCl → AgCl↓ + NaNO₃ (white silver chloride precipitate)
- Pb(NO₃)₂ + 2KI → PbI₂↓ + 2KNO₃ (bright yellow lead iodide — the “golden rain” demonstration)
- Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑ (gas formed)
Whether a precipitate forms is predicted by the solubility rules.
How to spot it: two ionic compounds in, two ionic compounds out, with the cations and anions exchanged.
5. Combustion
A substance reacts rapidly with oxygen, releasing heat and usually light. When a hydrocarbon (or anything made of C, H and O) burns completely, the products are always carbon dioxide and water.
- CH₄ + 2O₂ → CO₂ + 2H₂O
- C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
- C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (the same overall reaction as respiration)
With too little oxygen, incomplete combustion produces carbon monoxide and soot (carbon) as well — which is why faulty gas heaters are dangerous.
Metals burn too: 2Mg + O₂ → 2MgO, with a brilliant white flame.
How to spot it: O₂ as a reactant, lots of energy released; for fuels, CO₂ and H₂O as products.
6. Neutralization (acid-base)
An acid reacts with a base to form a salt and water.
- HCl + NaOH → NaCl + H₂O
- H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O
- 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂ (acid + carbonate also gives CO₂)
Technically this is a special kind of double displacement. The real reaction, stripped of spectator ions, is always H⁺ + OH⁻ → H₂O. See acid-base definitions.
7. Redox reactions: a category that overlaps
Oxidation-reduction isn’t a separate pattern so much as a property many reactions have: electrons move from one substance to another, and oxidation numbers change. All combustion reactions and single displacement reactions are redox; so are many syntheses and decompositions. Double displacement and neutralization reactions usually are not. See oxidation and reduction.
Quick identification guide
| Look for | Type |
|---|---|
| Many → one | Synthesis |
| One → many | Decomposition |
| Element + compound → element + compound | Single displacement |
| Two ionic compounds swap ions | Double displacement |
| Something + O₂ → CO₂ + H₂O (+ heat) | Combustion |
| Acid + base → salt + water | Neutralization |
Quick answers
Can a reaction be more than one type? Yes. 2H₂ + O₂ → 2H₂O is synthesis, combustion and redox all at once.
Why do some double displacement reactions not happen? If all possible products stay dissolved as ions, nothing has really changed — there’s no reaction.
Is photosynthesis a type of reaction? Overall, it’s the reverse of combustion — an endothermic redox reaction powered by light.
Useful tools
Once you know the products, the equation balancer finds the coefficients, and how to balance chemical equations shows the method by hand.
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