Types of Chemical Reactions: Synthesis and Decomposition

Contributor: Hannah Brooks. Lesson ID: 12908

Q. Where is Beethoven? A. In his grave, decomposing. Decomposition is the opposite of synthesis, like that was the opposite of a joke. Learn how atoms come together and separate to form reactions.



learning style
personality style
Grade Level
High School (9-12)
Lesson Type
Quick Query

Lesson Plan - Get It!

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List as many objects or ideas that are opposites that you can think of. Ready? Go!

Left and right, up and down, and fire and water — all of these are opposites that exist in our world.

  • Did you know that some chemical reactions are opposites of one another?

In this lesson, you will learn about synthesis and decomposition reactions that run the opposite way.

The word "synthesis" means to build larger substances from smaller materials.

wooden toy blocks

In a synthesis chemical reaction, two elements combine into a molecule or compound. Synthesis reactions follow this pattern: A + B —> AB. Notice that the two elements become chemically bonded during the chemical reaction.

An example is the synthesis of potassium bromide: K + Br —> KBr.

Decomposition reactions run the opposite direction, breaking down a compound into two elements: AB —> A + B.

When water decomposes, it breaks down into hydrogen and oxygen. The reaction looks like:

H2O —> H2 + O2

water droplet creating a ripple

Notice that the elements involved in the decomposition of water have subscripts of 2. This is an important characteristic of a group of elements located on the periodic table, called diatomic elements. The diatomic elements are nitrogen (N), oxygen (O), fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and hydrogen (H).

periodic table

When found in the elemental form, these elements exist as two bonded atoms. For example, oxygen would exist as an oxygen molecule with two oxygen atoms.

image of oxygen molecule

This is why you might notice some elements written with subscripts as single elements, while others have none. When you use these elements in a chemical reaction, you must write them with a subscript of 2.

An example is the synthesis of sodium chloride: Na + Cl2 —> NaCl. Remember that you must use an arrow, because it illustrates the chemical reaction occurring.

In order to balance the equation, you add coefficients to the reaction: 2Na + Cl2 —> 2 NaCl

Remember, when you are working with ionic compounds — those involving metals — you have to crisscross the charges to determine the chemical formula. Review Ionic Chemical Bonds, found in the right-hand sidebar under Related Lessons, to practice writing formulas.

Synthesis and decomposition reactions are the simplest types of chemical reactions because they involve the fewest elements. Summarize the difference in synthesis and decomposition reactions in two sentences. Synthesis reactions take elements and build them into a larger compound, while decomposition is the opposite, breaking compounds down into elements.

In the Got It? section, you will practice identifying synthesis and decomposition reactions.

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