When predicting the products of a reaction from known reactants, it is very helpful to determine what type of reaction is occurring. If there is just one reactant, the reaction is a decomposition reaction. The products are a result of breaking down the reactant into its elements. If there is oxygen and a hydrocarbon in the reactant, then the reaction is most likely combustion. Combustion always produces carbon dioxide and water. If the reactant contains two ionic substances, the reaction is double replacement. If the reactant has a compound and an element then the reaction is most likely a single replacement reaction. If the single element is a metal, it can replace the metal in the compound. If the single element is a nonmetal, it can replace the nonmetal in the compound. How will you know if you have a synthesis reaction? Although you could have many different scenarios of complicated synthesis reactions, you are expected to predict the product of a synthesis reaction if the reactants are two elements.