Reaction Definition in Chemistry

A reaction or chemical reaction is a chemical change which forms new substances. In other words, reactants react to form products that have a different chemical formula. Indications a reaction has occurred include temperature change, color change, bubble formation, and/or precipitate formation.

Chemical Reactions Take Different Forms

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What Is a Chemical Reaction?

By Anne Marie Helmenstine, Ph.D.

The major types of chemical reaction are:

• Synthesis or Direct Combination Reaction - Reactants form a more complex product.
• Decomposition or Analysis Reaction - A reactant breaks into two or more smaller products.
• Single Displacement or Replacement Reaction - Also called a substitution reaction, this occurs when the ion from one reactant changes place with another.
• Double Displacement or Replacement Reaction - Also called a metathesis reaction, this occurs when both cations and anions of the reactants trade places to form products.

While some reactions involve a change in the state of matter (e.g., liquid to gas phase), a phase change is not necessarily an indicator of a reaction. For example, melting ice into water is not a chemical reaction because the reactant is chemically identical to the product.

Reaction Example: The chemical reaction H ₂(g) + ½ O ₂(g) → H ₂O(l) describes the formation of water from its elements.