Types of Reactions Types of Reactions There are four types of reactions. Synthesis Decomposition Single replacement Double replacement Occurs when two substances unite Synthesis A + B AB Synthesis 2 Mg + O2 2 MgO magnesium(element) oxygen(element) magnesium oxide(compound) 4 Occurs when a substance splits into parts Decomposition AB A + B Decomposition hydrogen(element) oxygen(element) water(compound) 2 H2O 2 H2 + O2 6 Occurs when a more active substance replaces one part of a compound that is less active Single Replacement Y X + X Y Z Z + Single Replacement hydrogen(element) potassiumhydroxide(compound) water(compound) potassium(element) 2 K + 2 H2O H2 + 2 KOH 8 These reactions are also called displacement or substitution reactions. Single Replacement