What is an example of a direct combination chemical reaction in nonliving systems?

An example of a direct combination chemical reaction in nonliving systems is the formation of water from hydrogen and oxygen gases. When two hydrogen molecules (H₂) react with one oxygen molecule (O₂), they combine to form two molecules of water (H₂O). This reaction is represented by the equation 2H₂ + O₂ → 2H₂O and is exothermic, releasing energy in the process.