What type of reaction is characterized by the combination of two or more substances?

A combination reaction is defined by the process in which two or more reactants come together to form a single product. This type of reaction is also known as a synthesis reaction. The essence of a combination reaction lies in the unification of elements or compounds, resulting in a new compound that incorporates the properties of the reactants in a specific stoichiometric ratio.

For instance, when hydrogen gas reacts with oxygen gas, they combine to form water, a single product from two reactants. This showcases the hallmark characteristic of a combination reaction—multiple simple reactants converge to form a more complex product.

In contrast, the other reaction types listed operate under different mechanisms. Decomposition reactions involve a single compound breaking down into two or more simpler products. Displacement reactions feature an exchange where an element in a compound is replaced by another element. Redox reactions encompass a broader category of reactions involving the transfer of electrons, resulting in changes in oxidation states, which may or may not be combination reactions. Thus, the defining nature of the combination reaction stands out distinctly in this context.