Chemical reactions · Topic 7 of 15

Types of Chemical Reactions

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Chemistry · reaction types

Types of Chemical Reactions

Five patterns: synthesis joins, decomposition splits, single replacement swaps one element, double replacement swaps partners, combustion burns with O₂.

01 · JoinsSynthesis: A + B → AB

02 · Swaps partnersDouble: AB + CD → AD + CB

03 · BurnsFuel + O₂ → CO₂ + H₂O

What it is

Synthesis: A + B → AB (two or more substances make one product) must know

Decomposition: AB → A + B (one compound splits into simpler ones) must know

Single replacement: A + BC → AC + B (an element swaps in) must know

Double replacement: AB + CD → AD + CB (ions trade partners) must know

Combustion: fuel + O₂ → heat and light; hydrocarbon → CO₂ + H₂O must know

Precipitate = insoluble solid formed from solutions (marked (s))

Decomposition usually needs energy in: heat, light or electricity

The rules

Double replacement goes when it forms a precipitate, a gas or water

Combustion always has O₂ as a reactant and releases energy

Worked examples

2NaN₃ → 2Na + 3N₂ = decomposition (one reactant splits)

Zn + CuSO₄ → ZnSO₄ + Cu = single replacement

BaCl₂ + Na₂SO₄ → BaSO₄(s) + 2NaCl = double replacement

Don't confuse

Acid + base → salt + water is a double replacement (see Acids, Bases & pH)

Single or double replacement?

Commonly confused

	Single replacement	Double replacement
Pattern	A + BC → AC + B	AB + CD → AD + CB
Reactants	An element + a compound	Two compounds
What swaps	One element takes another's place	Cations trade anion partners
Sign it happened	A new free element forms	A precipitate, a gas or water forms

Common mistakes

One reaction can fit two types: 2H₂ + O₂ → 2H₂O is synthesis and combustion

Also know

One product → synthesis; one reactant → decomposition

Element + compound → single; compound + compound → double

More detail

Show 3 more facts

Single replacement needs a more reactive element (activity series)

Single replacement and combustion are redox reactions

Metals replace metals; halogens replace halogens

Pearl

Count the boxes: one product means synthesis, one reactant means decomposition, O₂ burning a fuel means combustion.

Sources · LibreTexts Introductory Chemistry (CK-12) 11.4 Combination Reactions · LibreTexts Introductory Chemistry (CK-12) 11.5 Decomposition Reactions · LibreTexts Introductory Chemistry (CK-12) 11.6 Combustion Reactions · LibreTexts Introductory Chemistry (CK-12) 11.7 Single Replacement Reactions · LibreTexts Introductory Chemistry (CK-12) 11.9 Double Replacement Reactions · OpenStax Chemistry 2e §4.2 Classifying Chemical Reactions · LibreTexts Introductory Chemistry (CK-12) 11.8 Activity Series · Union Test Prep — HESI Chemistry study guide, page 2 (reaction types) · NurseHub — HESI A2 Chemistry course outline (Chemical Reactions lesson) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

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