What do we call a reaction that combines substances to create new compounds?

A reaction that combines substances to create new compounds is known as a combination reaction. In this type of reaction, two or more reactants join together to form a single product. This process is fundamental in chemistry because it illustrates how different elements or compounds can unite to create something new, often involving the making of chemical bonds.

For example, when hydrogen gas reacts with oxygen gas, they combine to form water, which is a new compound. This process not only highlights the principles of chemical bonding but also underscores the conservation of mass, where the total mass of the reactants equals the total mass of the products.

The other types of reactions mentioned do not represent the combination of substances to form new compounds. Decomposition reactions involve a single compound breaking down into two or more simpler products, while endothermic and exothermic reactions refer to the energy changes that occur during chemical reactions rather than the combination of reactants. These concepts provide clarity on the specific nature of combination reactions as a distinct category in chemical processes.