Science, Tech, Math › Science Chain Reaction Definition in Chemistry and Physics What Is a Chain Reaction in Science? Share Flipboard Email Print In a chain reaction, one action leads to another and another. JamesBrey, 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 July 09, 2019 In science, a chain reaction is a series of reactions where the products contribute to the reactants of another reaction without outside influence. The idea of chain reactions was introduced by German chemist Max Bodenstein in 1913 in reference to chemical reactions. Chain Reactions Examples A nuclear chain reaction is a fission reaction where the neutrons generated by the fission process go on and initiate fission in other atoms. The chemical reaction between hydrogen gas and oxygen gas to form water is another example of a chain reaction. In the reaction, one hydrogen atom is replaced by another as well as two OH radicals. The propagation of the reaction can lead to an explosion. Chain Reaction Steps A typical chain reaction follows a sequence of steps: Initiation: Active particles form that serve as the basis for the reaction.Propagation: Active particles react with each other and may serve as catalysts to perpetuate the cycle.Termination: The active particles lose their activity, slowing and ending the reaction.