The reaction that occurs when two or more substances combine to form a single compound is called a synthesis reaction or a combination reaction.

Here's a general representation:

A + B → AB

Where:

* A and B are the reactants (the substances that combine)

* AB is the product (the single compound formed)

Examples of synthesis reactions:

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

* Burning of magnesium: 2Mg (s) + O₂ (g) → 2MgO (s)

* Formation of sodium chloride: 2Na (s) + Cl₂ (g) → 2NaCl (s)

In synthesis reactions, energy is typically released (exothermic reaction), but sometimes energy is required (endothermic reaction) for the reaction to occur.