By Lan Luo
Updated Aug 30, 2022

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Chemical reactions are integral to modern technology and everyday life—from powering engines to brewing beverages. They also drive natural processes such as photosynthesis and rock weathering.

Three Broad Categories of Reactions

Physical reactions involve only a change in state or physical arrangement, with no alteration to the chemical identity of the substances. For example, freezing water:
H₂O(l) → H₂O(s)

Chemical reactions transform reactants into new products through the rearrangement of atoms. A classic example is dissolving CO₂ in water to form carbonic acid:
CO₂(g) + H₂O(l) → H₂CO₃(aq)

Nuclear reactions involve changes to atomic nuclei, such as Rutherford’s transmutation of nitrogen:
¹⁴N + α → ¹⁷O + p

The Six Core Chemical Reaction Types

Synthesis (Combination) Reaction

Two or more reactants combine to produce a single, more complex product. General form:
A + B → AB

Examples:
Fe(s) + S(s) → FeS(s)
2Na(s) + Cl₂(g) → 2NaCl(s)

Decomposition Reaction

A single complex reactant breaks into two or more simpler products. General form:
AB → A + B

Examples:
H₂O(l) → H₂(g) + O₂(g) (electrolysis)
H₂CO₃(aq) → H₂O(l) + CO₂(g) (thermal decomposition)

Single Displacement (Replacement) Reaction

An element displaces another from a compound. General form:
A + BC → AC + B

Examples:
Mg(s) + 2HCl(aq) → H₂(g) + MgCl₂(aq)
Mg(s) + 2H₂O(l) → H₂(g) + Mg(OH)₂(aq)

Double Displacement (Metathesis) Reaction

Cations and anions swap partners, forming two new compounds. General form:
AB + CD → AD + CB

Examples:
BaCl₂ + MgSO₄ → BaSO₄ + MgCl₂
Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃

Combustion Reaction

An exothermic redox process where a fuel reacts with oxygen, producing mainly CO₂ and H₂O. Complete combustion:
4CxHy + (4x+y)O₂ → 4xCO₂ + 2yH₂O + heat

Specific examples:
CH₄ + 2O₂ → CO₂ + 2H₂O + heat (≈891 kJ mol⁻¹)
C₁₀H₈ + 12O₂ → 10CO₂ + 4H₂O + heat
CH₃OH + O₂ → CO₂ + 2H₂O + heat

Incomplete combustion occurs when O₂ is limited, producing CO, soot, or other by‑products.

Acid‑Base Reaction

A neutralization process where an acid reacts with a base to form a salt and water. It is a special case of double displacement:
HA + BOH → BA + H₂O

Examples:
Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O
CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂

Why Understanding These Types Matters

Recognizing the reaction pattern aids chemists and engineers in predicting products, optimizing conditions, and ensuring safety—especially in industrial combustion or pharmaceutical synthesis.