The Five Types of Chemical Reactions

Chemical reactions can look intimidating, but most of the ones taught in an introductory course fall into just five recognizable patterns. Learning to recognize the pattern is often more useful than memorizing any single equation.

1. Synthesis: A + B → AB

Two (or more) substances combine to form a single, more complex product.

Example: 4 Fe + 3 O2 → 2 Fe2O3 (iron rusting is a synthesis reaction)

2. Decomposition: AB → A + B

The reverse of synthesis — a single compound breaks down into two or more simpler substances, usually with heat, light, or electricity as the trigger.

Example: 2 H2O2 → 2 H2O + O2 (hydrogen peroxide breaking down into water and oxygen)

3. Single Replacement: A + BC → AC + B

One element swaps places with another element within a compound. This only happens if the "swapping in" element is more reactive than the one it displaces.

Example: Zn + 2 HCl → ZnCl2 + H2 (zinc displaces hydrogen from hydrochloric acid)

4. Double Replacement: AB + CD → AD + CB

Two compounds trade partners — the positive part of one pairs with the negative part of the other, and vice versa. These often happen in solution and produce a solid precipitate, a gas, or water.

Example: BaCl2 + Na2SO4 → BaSO4 + 2 NaCl (forms solid barium sulfate)

5. Combustion: Hydrocarbon + O2 → CO2 + H2O

A hydrocarbon (or similar fuel) reacts with oxygen, releasing energy and producing carbon dioxide and water. This is the reaction behind burning fuel, from a candle to a car engine.

Example: CH4 + 2 O2 → CO2 + 2 H2O (methane combustion)

Why the pattern matters more than the specific equation

You'll never memorize every possible chemical equation — there are far too many. But once you can recognize which pattern a reaction follows, you know what kind of products to expect, even for a reaction you've never seen before.

Try it yourself: the Worksheet Studio in Science Lab lets you generate practice problems for exactly these five reaction types — either asking you to balance them, identify the type, or both — with an instant answer key.