Science, Tech, Math › Science Synthesis Reaction Description Plus Examples Two or more simple substances combine to form more complex products Share Flipboard Email Print In a synthesis reaction, simple reactants combine to form a more complex product. Rafe Swan / Getty Images Science Chemistry Basics Chemical Laws 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 May 10, 2019 While there are many types of chemical reactions, they all fall into at least one of four broad categories: synthesis reactions, decomposition reactions, single displacement reactions, and double displacement reactions. A synthesis reaction or direct combination reaction is a type of chemical reaction in which two or more simple substances combine to form a more complex product. The reactants may be elements or compounds, while the product is always a compound. General Form of Synthesis Reactions The general form of a synthesis reaction is: A + B → AB Examples of Synthesis Reactions Here are some examples of synthesis reactions: Water:2 H2(g) + O2(g) → 2 H2O(g)Carbon dioxide:2 CO(g) + O2(g) → 2CO2(g)Ammonia:3 H2(g) + N2(g) → 2 NH3(g)Aluminum oxide:4 Al(s) + 3 O2(g) → 2 Al2O3(s)Iron sulfide:8 Fe + S8 → 8 FeSPotassium chloride:2 K(s) + Cl2(g) → 2 KCl(s) Recognizing Synthesis Reactions The hallmark of a synthesis reaction is that a more complex product is formed from the reactants. One easy-to-recognize type of synthesis reaction occurs when two or more elements combine to form a compound. The other type of synthesis reaction happens when an element and a compound combine to form a new compound. Basically, to identify this reaction, look for a product that contains all the reactant atoms. Be sure to count the number of atoms in both the reactants and the products. Sometimes when a chemical equation is written, "extra" information is given that might make it hard to recognize what is going on in a reaction. Counting numbers and types of atoms makes it easier to identify reaction types.