A combination reaction or a synthesis reaction is a general category of a chemical reaction

Chemical reaction

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

 (the term usually refers to an inorganic chemical reaction), in which two or more reactants are chemically bonded

Chemical bond

A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...

 together to produce a single product

Product (chemistry)

Product are formed during chemical reactions as reagents are consumed. Products have lower energy than the reagents and are produced during the reaction according to the second law of thermodynamics. The released energy comes from changes in chemical bonds between atoms in reagent molecules and...

. When two or more substances combine to form a single product, it is known as a combination reaction. Many elements react with one another in this fashion to form compounds.

This reaction is usually exothermic

Exothermic

In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...

 because when the bond forms between the reactants, heat is released.

A combination reaction can be of three types:

Type	Example
a) Between two elements	C + O2 → CO2	Carbon Carbon Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...  completely burnt in oxygen Oxygen Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...  yields carbon dioxide Carbon dioxide Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
b) Between two compounds	CaO + H2O → Ca(OH)2	Calcium oxide Calcium oxide Calcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....  (lime) combined with water gives calcium hydroxide Calcium hydroxide Calcium hydroxide, traditionally called slaked lime, is an inorganic compound with the chemical formula Ca2. It is a colourless crystal or white powder and is obtained when calcium oxide is mixed, or "slaked" with water. It has many names including hydrated lime, builders lime, slack lime, cal, or...  (slaked lime)
c) Between an element and a compound	O + H2O → H2O2	Oxygen Oxygen Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...  (an element) combined with water (a compound) yields hydrogen peroxide Hydrogen peroxide Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...

When a combination reaction occurs between a metal and a nonmetal the product is an ionic solid. An example could be Lithium + Sulphur → Lithium Sulphide

When magnesium metal burns in air to produce magnesium oxide, a combination reaction occurs between magnesium and the diatomic oxygen molecule. This specific combination reaction produces the bright flame generated by flares.