What Is a Chemical Reaction? Definition and Examples

What Is a Chemical Reaction
A chemical reaction is a process that changes the chemical identity of the starting materials.

Chemical reactions are the backbone of chemistry and, arguably, life itself. Understanding what a chemical reaction is, how to represent it, how to categorize it, and how to distinguish it from a physical change is vital.

What Is a Chemical Reaction?

A chemical reaction is a process in which the chemical structure of a substance changes, leading to the formation of a new substance with different properties. In other words, the reactants convert into products through the breaking and formation of chemical bonds.

Describing Chemical Reactions Using Chemical Equations

A chemical equation is a symbolic representation of a chemical reaction. Reactants are written on the left-hand side, and products on the right, separated by an arrow indicating the direction of the reaction. Combinations of coefficients, element symbols, subscripts, and superscripts indicate the chemical formulas of the reactants and products and their quantities. For each chemical formula, the cation (positive-charged part) of a compound gets listed before the anion (negative-charged part). For example, you write NaCl for sodium chloride rather than ClNa.

A balanced chemical equation follows conservation of mass and charge. There are exactly the same number of atoms of each element on both the reactant and product sides of the equation. The net electrical charge is also the same for both sides of the equation.

Examples of Chemical Reactions

For example, here are some chemical reactions represented as chemical equations:

  • The formation of water from hydrogen and oxygen: 2H2 + O2 2H2O
  • The combustion of methane: CH4 + 2O2 CO2 + 2H2O
  • The decomposition of calcium carbonate: CaCO3 CaO + CO2

How to Recognize a Chemical Reaction

Not all changes involving matter are chemical reactions. A chemical reaction is a chemical change, which means the starting materials are chemically different from the ending materials. In contrast, matter also changes form via physical changes. But, in a physical change, the chemical identity of matter does not change.

For example, when you melt an ice cube into liquid water, the chemical identity of the ice and the water is the same (H2O). Melting (and any other phase transition) is an example of a physical change. No chemical reaction occurs. However, when you combine baking soda (NaHCO3) and vinegar (CH3COOH), the two chemical undergo a chemical reaction that produces sodium acetate (NaC2H3O2), water (H2O), and carbon dioxide (CO2).

You can’t see the atoms and molecules in action and in the examples of melting ice and reacting baking soda and vinegar, you start with a transparent substance and end with one. So, how do you know which is a physical change and which is a chemical reaction? There are several indicators of a chemical change:

  • Color change
  • Forming a gas or bubbles
  • Forming a precipitate
  • Temperature change
  • Releasing or absorbing light or sound
  • Irreversibility (Most chemical changes are irreversible, while most physical changes are reversible.)
  • Changing chemical properties

Melting ice is reversible and does not really meet the other criteria for a chemical change, so it is a physical change. Mixing baking soda and vinegar results in bubbles, a temperature change, and new chemical properties.

Types of Chemical Reactions

There are many different types of chemical reactions, but there are four main classes:

Synthesis (Combination) Reactions

  • Description: Two or more substances combine to form a single product.
  • General Reaction: A + B → AB
  • Example: N2 + 3H2 → 2NH3

Decomposition Reactions

  • Description: A single compound breaks down into two or more simpler substances.
  • General Reaction: AB → A + B
  • Example: 2H2O → 2H2 + O2

Single-Replacement Reactions

  • Description: One element replaces another element in a compound.
  • General Reaction: A + BC → AC + B
  • Example: Zn + 2HCl → ZnCl2 + H2

Double-Replacement Reactions

  • Description: The cations and anions of two different molecules switch places.
  • General Reaction: AB + CD → AD + CB
  • Example: AgNO3 + NaCl → AgCl + NaNO3

Other Types of Reactions

There are many other types of reactions, such as:

  • Redox Reactions: Involves electron transfer.
  • Acid-Base Reactions: Involves the transfer of a proton.
  • Complexation Reactions: Formation of complex ions.
  • Polymerization: Formation of polymers from monomers.

Importance of Chemical Reactions

Chemical reactions are at the heart of chemistry. Understanding their mechanisms, types, and representations helps us grasp more complex concepts and applications. From the combustion that powers our cars to the metabolic reactions that keep us alive, chemical reactions are indispensable to our daily lives. Applications include:

  • Medication formulation
  • Making cleaners
  • Making disinfectants
  • Waste treatment
  • Food processing
  • Energy production
  • Material design


  • Atkins, Peter W.; Julio de Paula (2006). Physical Chemistry (4th ed.). Weinheim: Wiley-VCH. ISBN 978-3-527-31546-8.
  • IUPAC (1997). Compendium of Chemical Terminology (the “Gold Book”) (2nd ed.). Oxford: Blackwell Scientific Publications. ISBN 0-9678550-9-8. doi:10.1351/goldbook
  • Wintterlin, J. (1997). “Atomic and Macroscopic Reaction Rates of a Surface-Catalyzed Reaction”. Science. 278 (5345): 1931–4. doi:10.1126/science.278.5345.1931
  • Zumdahl, Steven S.; Zumdahl, Susan A. (2000). Chemistry (5th ed.). Houghton Mifflin. ISBN 0-395-98583-8.