Science, Tech, Math › Science Synthesis Reaction Definition and Examples Overview of a Synthesis or Direct Combination Reaction Share Flipboard Email Print In a synthesis reaction, two or more reactants combine to form a more complex product. shapecharge /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 December 02, 2019 A synthesis reaction or direct combination reaction is one of the most common types of chemical reaction. In a synthesis reaction, two or more chemical species combine to form a more complex product: A + B → AB. In this form, a synthesis reaction is easy to recognize because you have more reactants than products. Two or more reactants combine to make one larger compound. One way to think of synthesis reactions is that they are the reverse of a decomposition reaction. Synthesis Reaction Examples In the simplest synthesis reactions, two elements combine to form a binary compound (a compound made of two elements). The combination of iron and sulfur to form iron (II) sulfide is an example of a synthesis reaction: 8 Fe + S8 → 8 FeS Another example of a synthesis reaction is the formation of potassium chloride from potassium and chlorine gas: 2K(s) + Cl2(g) → 2KCl(s) As in these reactions, it's common for a metal to react with a nonmetal. One typical nonmetal is oxygen, as in the everyday synthesis reaction of rust formation: 4 Fe (s) + 3 O2 (g) → 2 Fe2O3 (s) Direct combination reactions aren't always just simple elements reacting to form compounds: Another everyday synthesis reaction, for example, is the reaction that forms hydrogen sulfate, a component of acid rain. Here, the sulfur oxide compound reacts with water to form a single product: SO3 (g) + H2O (l) → H2SO4 (aq) Multiple Products So far, the reactions you have seen have only one product molecule on the right-hand side of the chemical equation. Let's take a look at more complex reactions with multiple products. For example, the overall equation for photosynthesis: CO2 + H2O → C6H12O6 + O2 The glucose molecule is more complex than either carbon dioxide or water. Remember, the key to identifying a synthesis or direct combination reaction is to recognize two or more reactants form a more complex product molecule. Predictable Products Certain synthesis reactions form predictable products. For example: Combining two pure elements will form a binary compound.A metallic oxide and carbon dioxide will form a carbonate.Binary salts combined with oxygen form a chlorate.