The chemical reaction in which two or more substances undergo union to form a complex substance is called a synthesis reaction or a combination reaction.

Here's a breakdown:

* Synthesis: This term implies the creation of a new, more complex substance from simpler starting materials.

* Combination: This term emphasizes the joining together of two or more reactants to form a single product.

General Form:

A + B → AB

Examples:

* Formation of water: 2H₂ (g) + O₂ (g) → 2H₂O (l)

* Rusting of iron: 4Fe (s) + 3O₂ (g) → 2Fe₂O₃ (s)

* Combustion of methane: CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l)

Key Features of Synthesis Reactions:

* Two or more reactants combine.

* One product is formed.

* The product is usually more complex than the reactants.

* Energy is often released (exothermic), but some reactions are endothermic.

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