Types of Reactions

A great deal of information about reactions has gone forth up to this point. By now, a solid familiarity with how substances react should exist.

One additional concept related to the reaction of substances is being able to describe them as one of several types of reactions. Based on how the elements combine, separate, or reorganize, reactions can be differentiated into one of five reaction types.

Synthesis

When two (or more) substances combine to form a single new substance, the type of reactions is called synthesis.

What this looks like:

A + B --> AB 

"Two smaller things join together to become one bigger thing." 

In synthesis, the substances that combine (the reactants) can be either elements or compounds.

The product will always be a compound.

There are many examples of synthesis. For example, 2H2 + O2 --> 2H2O represents the synthesis of water from hydrogen and oxygen. Likewise, Na + Cl --> NaCL represents the synthesis of salt.

Decomposition

Decomposition occurs when a compound breaks down into two or more substances.

What this looks like:

AB --> A + B

"One bigger thing breaks up into two (or more) smaller things."

In decomposition, the substance that comes apart (the reactants) must be a compound.

The product can be compounds or elements.

An example of decomposition can be seen in the reaction CaCO3 --> CaO + CO2 where the reactant breaks down into two different products—in this case a compound decomposes into two different compounds.

Single Replacement

Single replacement occurs when there are multiple reactants that combine in a way that something in one compound is replaced by the other reactant.

"A single thing replaces something in a compound." 

What this looks like:

AB + C --> A + BC

A + BC -> AB + C

In single replacement, at least one of the reactants and one of the products has to be a compound.

An example of single replacement can be seen in the reaction

2K + 2H2O --> H2 + 2KOH

where the K takes the place of one H. Here is another example: 

2AG + H2S --> Ag2S + H2 

where the Ag takes the place of the H. 

Double Replacement

Double replacement occurs when there are multiple reactants that combine in a way that something in each reactant is replaced by something from the other reactant. In essence, parts each compound swap places.

"Two things in different compounds swap places."

What this looks like:

AB + CD --> AC + BD

In double replacement, the reactants and products are compounds.

An example of double replacement can be seen in the reaction:

BaCl2 + Na2SO4 --> BaSO4 + 2NaCl

where the Ba and Na switch places (as demonstrated by the colors).

Combustion

Combustion exists whenever a substance reacts with oxygen and often produces heat and light. This is usually called burning. The key is that it is a reaction wherein something combines with oxygen. 

"Rapid chemical combination of a substance with oxygen involving the production of heat and light."

There are many, many examples.

See also: https://en.wikipedia.org/wiki/Combustion

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These five ways of classifying reactions provide a convenient way to describe reactions between substances. It is a very useful way to get an idea of what is happening when considering chemical reactions.