Thousands of reactions follow a handful of patterns. Once an equation is written and balanced, the pattern of who ends up with whom tells you its type, and knowing the types helps you predict products.
Key idea
Five common types: synthesis (two or more substances join into one), decomposition (one substance breaks apart), single replacement (an element takes the place of another in a compound), double replacement (two compounds swap partners) and combustion (a fuel reacts with oxygen, giving carbon dioxide and water).
| Type | Pattern | Example |
|---|---|---|
| synthesis | A + B → AB | $2\mathrm{Na} + \mathrm{Cl}_{2} \to 2\mathrm{NaCl}$ |
| decomposition | AB → A + B | $2\mathrm{H}_{2}\mathrm{O}_{2} \to 2\mathrm{H}_{2}\mathrm{O} + \mathrm{O}_{2}$ |
| single replacement | A + BC → AC + B | $\mathrm{Zn} + 2\mathrm{HCl} \to \mathrm{ZnCl}_{2} + \mathrm{H}_{2}$ |
| double replacement | AB + CD → AD + CB | $\mathrm{AgNO}_{3} + \mathrm{NaCl} \to \mathrm{AgCl} + \mathrm{NaNO}_{3}$ |
| combustion | fuel + O₂ → CO₂ + H₂O | $\mathrm{CH}_{4} + 2\mathrm{O}_{2} \to \mathrm{CO}_{2} + 2\mathrm{H}_{2}\mathrm{O}$ |
Why it works
The types describe what happens to the partners. Atoms either join, split up, trade places one at a time or swap in pairs. Combustion gets its own name because it is so important: oxygen reacts with a fuel made of carbon and hydrogen, and a lot of energy comes out as heat and light.
Worked example: A metal in an acid
Classify $$\mathrm{Zn} + 2\mathrm{HCl} \to \mathrm{ZnCl}_{2} + \mathrm{H}_{2}$$
- One reactant is an element (zinc) and one product is an element (hydrogen).
- Zinc took hydrogen's place next to the chloride ions.
Answer: Single replacement.
Worked example: A fuel that contains oxygen
Classify $$\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH} + 3\mathrm{O}_{2} \to 2\mathrm{CO}_{2} + 3\mathrm{H}_{2}\mathrm{O}$$
- Ethanol is a fuel made of carbon, hydrogen and some oxygen.
- It reacts with $\mathrm{O}_{2}$ and gives only carbon dioxide and water.
Answer: Combustion. Oxygen inside the fuel does not change the type.
Common mistake
Sorting by surface pattern alone. "Two compounds react" does not always mean double replacement: $\mathrm{CaO} + \mathrm{H}_{2}\mathrm{O} \to \mathrm{Ca}(\mathrm{OH})_{2}$ makes one product, so it is synthesis, and $\mathrm{CH}_{4} + 2\mathrm{O}_{2} \to \mathrm{CO}_{2} + 2\mathrm{H}_{2}\mathrm{O}$ is combustion. And a fuel that contains oxygen, such as ethanol, still burns in combustion. Check the products: one product means synthesis; carbon dioxide and water from a fuel and oxygen means combustion.
Some reactions fit two types. Magnesium burning in oxygen is both a synthesis and a combustion, and an acid reacting with a hydroxide base is a double replacement that is also called a neutralization.