Which type of reaction involves the combination of two or more substances to form a new compound?

A synthesis reaction, also known as a combination reaction, is characterized by the process in which two or more reactants combine to form a single, more complex product. This type of reaction is fundamental in chemistry as it illustrates how two or more simple substances can interact to create a new substance with different properties.

For example, when hydrogen gas combines with oxygen gas, they react to form water, which is a new compound. The general form of a synthesis reaction can be represented as:

A + B → AB

In this scenario, A and B are the reactants that come together to form the compound AB. Synthesis reactions are crucial in both laboratory settings and industrial applications, serving as the basis for creating biomolecules, materials, and various chemicals.

In contrast, decomposition reactions involve a single compound breaking down into two or more simpler substances. Single replacement reactions occur when one element replaces another element in a compound, and double replacement reactions involve the exchange of ions between two compounds. Thus, the distinct nature of the synthesis reaction lies in its formation of a new compound from multiple starting materials.