Simple Chemical Reactions

By
Anne Marie Helmenstine, Ph.D.
Anne Marie Helmenstine, Ph.D.
Chemistry Expert
  • Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
  • B.A., Physics and Mathematics, Hastings College
Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.
Learn about our Editorial Process
Updated on May 09, 2019

Chemical reactions are evidence a chemical change is occurring. The starting materials change into new products or chemical species. How do you know a chemical reaction has taken place? If you observe one or more of the following, a reaction may have occurred:

  • color change
  • gas bubbles
  • formation of a precipitate
  • temperature change (although physical changes may also involve temperature change)

While there are millions of different reactions, most can be classified as belonging to one of 5 simple categories. Here is a look at these 5 kinds of reactions, with the general equation for each reaction and examples.

Synthesis Reaction or Direct Combination Reaction

This is the general form of a synthesis reaction.
This is the general form of a synthesis reaction. Todd Helmenstine

One of the main types of chemical reactions is a synthesis or direct combination reaction. As the name implies, simple reactants make or synthesize a more complex product. The basic form of a synthesis reaction is:

A + B → AB

A simple example of a synthesis reaction is the formation of water from its elements, hydrogen, and oxygen:

2 H2(g) + O2(g) → 2 H2O(g)

Another good example of a synthesis reaction is the overall equation for photosynthesis, the reaction through which plants make glucose and oxygen from sunlight, carbon dioxide, and water:

6 CO2 + 6 H2O → C6H12O6 + 6 O2

Decomposition Chemical Reactions

This is the general form of a decomposition reaction.
This is the general form of a decomposition reaction. Todd Helmenstine

The opposite of a synthesis reaction is a decomposition or analysis reaction. In this type of reaction, the reactant breaks down into simpler components. A telltale sign of this reaction is that you have one reactant, but multiple products. The basic form of a decomposition reaction is:

AB → A + B

Breaking water into its elements is a simple example of a decomposition reaction:

2 H2O → 2 H2 + O2

Another example is the decomposition of lithium carbonate into its oxide and carbon dioxide:

Li2CO3 → Li2O + CO2

Single Displacement or Substitution Chemical Reactions

This is the general form of a single displacement reaction.
This is the general form of a single displacement reaction. Todd Helmenstine

In a single displacement or substitution reaction, one element replaces another element in a compound. The basic form of a single displacement reaction is:

A + BC → AC + B

This reaction is easy to recognize when it takes the form of:

element + compound → compound + element

The reaction between zinc and hydrochloric acid to form hydrogen gas and zinc chloride is an example of a single displacement reaction:

Zn + 2 HCl → H2 + ZnCl2

Double Displacement Reaction or Metathesis Reaction

This is the general form for a double displacement reaction.
This is the general form for a double displacement reaction. Todd Helmenstine

A double displacement or metathesis reaction is just like a single displacement reaction, except two elements replace two other elements or "trade places" in the chemical reaction. The basic form of a double displacement reaction is:

AB + CD → AD + CB

The reaction between sulfuric acid and sodium hydroxide to form sodium sulfate and water is an example of a double displacement reaction:

H2SO4 + 2 NaOH → Na2SO4 + 2 H2O

Combustion Chemical Reactions

This is the general form of a combustion reaction.
This is the general form of a combustion reaction. Todd Helmenstine

A combustion reaction occurs when a chemical, usually a hydrocarbon, reacts with oxygen. If a hydrocarbon is a reactant, the products are carbon dioxide and water. Heat is released, too. The easiest way to recognize a combustion reaction is to look for oxygen on the reactant side of a chemical equation. The basic form of a combustion reaction is:

hydrocarbon + O2 → CO2 + H2O

A simple example of a combustion reaction is the burning of methane:

CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)

More Types of Chemical Reactions

Although there are 5 main types of chemical reactions, other kinds of reactions also occur.
Although there are 5 main types of chemical reactions, other kinds of reactions also occur. Don Bayley, Getty Images

In addition to the 5 main types of chemical reactions, there are other important categories of reactions and other ways to classify reactions. Here are some more kinds of reactions:

  • acid-base reaction: HA + BOH → H2O + BA
  • neutralization reaction: acid + base → salt + water
  • oxidation-reduction or redox reaction: one atom gains an electron while another atom loses an electron
  • isomerization: structural arrangement of a molecule changes, although its formula remains the same
  • hydrolysis: AB + H2O → AH + BOH
Format
mla apa chicago
Your Citation
Helmenstine, Anne Marie, Ph.D. "Simple Chemical Reactions." ThoughtCo, Aug. 25, 2020, thoughtco.com/simple-chemical-reactions-607971. Helmenstine, Anne Marie, Ph.D. (2020, August 25). Simple Chemical Reactions. Retrieved from https://www.thoughtco.com/simple-chemical-reactions-607971 Helmenstine, Anne Marie, Ph.D. "Simple Chemical Reactions." ThoughtCo. https://www.thoughtco.com/simple-chemical-reactions-607971 (accessed April 19, 2024).