A chemical reaction is usually defined by chemical change which occurs. Four common chemical reactions include:

1. Single Replacement Reactions (a type of oxidation-reduction reactions)
2. Double replacement reactions (metathesis reactions)
3. Acid-base reactions
4. Synthesis Reactions

Single Replacement Reactions

In a single replacement reaction, electrons are transferred from one chemical species to another. The transfer of electrons results in a change in the nature and the charge of the species. The single replacement reaction is a form of an oxidation-reduction reaction. A real-world example of a single replacement reaction is the formation of rust on cars. In its simplest form, iron (Fe) in the car's metal body can oxidize forming iron oxide (Fe₂O₃), also known as rust. Chemically, the iron reacts with water and oxygen to form hydrated iron oxide.

To learn more about oxidation-reduction reactions, click here.

Double Replacement Reactions

Whether an ionic compound dissolves in water is determined by its solubility. Therefore, we use the solubility rules to decide if a solid will become a solution when mixed with water. The general solubility rules for common salts are as follows:

A reaction in which the chemical reactants exchange ions but do not change oxidation state is called a double replacement reaction. This type of reaction is characterized by the formation of a solid precipitate. (see the solubility rules above) For example, the reaction of aqueous silver nitrate (soluble) with aqueous sodium chloride forms solid silver chloride and aqueous sodium nitrate.

Acid-Base Reactions

According to Bronsted Lowery:

1. An acid is any compound which can donate a proton.
2. A base is any compound which can accept a proton.

In an acid-base reaction, an acid and a base will react to form a salt and water. During the reaction, a proton is donated by the acid to the base to yield water. The remaining ions form a salt.

Here, hydrochloric acid (a proton donor) reacts with sodium hydroxide (a proton acceptor) to form sodium chloride (a salt) and water.

Synthesis Reactions

A synthesis reaction is a chemical reaction in which a certain product is sought. For instance, one may mix salicylic acid with acetic anhydride to make aspirin (acetyl salicylic acid). The desired product here is aspirin. Though all of the above reactions are a form of a synthesis reactions, they are not usually performed because of the products formed.