Science, Tech, Math › Science Reaction Definition in Chemistry Share Flipboard Email Print GIPhotoStock / Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated January 06, 2019 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 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.