What type of reaction occurs when two or more substances combine to create a new compound?

A synthesis reaction occurs when two or more substances combine to form a new compound. This type of reaction is characterized by the merging of reactants to produce a more complex product. For example, when hydrogen gas reacts with oxygen gas, water is formed, demonstrating the basic principle of synthesis reactions in which simpler substances come together.

In contrast, a decomposition reaction involves a single compound breaking down into two or more simpler substances, which is the opposite of a synthesis reaction. A single-displacement reaction occurs when an element displaces another element in a compound, leading to the formation of a new compound and a different free element. Lastly, combustion reactions typically involve a substance reacting with oxygen, resulting in the release of energy, often in the form of heat and light, rather than the formation of a new compound from multiple reactants. Thus, the synthesis reaction is uniquely defined by the creation of new compounds through the combination of various reactants.