While there are a number of types of chemical reactions, there are four main kinds that each type falls into: synthesis reaction (also known as combination reaction), decomposition reaction, and single and double replacement reaction. Synthesis, which in this case means to join, results in two or more base substances binding together on a molecular level to create a single compound. In contrast, a decomposition reaction results in a breakdown of individual components to yield separate substances, much as the name implies. Both a single and double replacement reaction involve two reactants that produce two new products. The only difference between these reactions is the manner in which they replace (or displace) certain components within a compound.
There is a basic principle of physics that states that neither matter nor energy can be created or destroyed. While this axiom remains constant, however, the various substances that make up matter and the energy they represent can be altered. In fact, this is the recipe for a chemical reaction. Knowing this leads to another concept relative to physics: all chemical reactions initiate change in material substance by generating an exchange of energy. How that energy is exchanged defines the type of chemical reaction that has taken place.
The primary difference between a single and double replacement reaction is that, in a single replacement reaction, a free element is exchanged for another to yield a new compound and a new element. In a double replacement reaction, the components of two compounds are exchanged (or displaced) to form two new compounds.
Some further clarification about how each reaction is defined should be considered to avoid possible confusion. First, most scientists accept the idea that nearly every replacement reaction involves ionizing. The basis for this belief stems from the fact that both usually take place in an aqueous solution containing primary constituents that are in ion form. So they are often referred to as ion reactions.
There is another school of thought, however, that both replacement reactions should be classified as being de-ionizing since it could be argued that they each lead to a pair of ions becoming displaced during the chemical reaction. Therefore, each is additionally sometimes referred to as a displacement reaction.