Synthesis Reaction: Definition, Formula & Examples

Lesson Transcript
Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

Expert Contributor
Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

Synthesis reactions occur when multiple substances are joined, react, and result in a complex product. Learn the chemical formulas for different synthesis reactions, and examples of binary, and more complex reactions in cooking. Updated: 11/01/2021

What is a Synthesis Reaction?

A cronut and crookie, two very hilarious food terms, they surely took the world by storm when making their grand debut. Did you know those are great examples of products made from a synthesis reaction? It may seem odd to mix a food term with a science term but it is definitely true. Before we discuss how this is so, let's go over what a synthesis reaction is.

A synthesis reaction is the joining together of two reactants, or compounds, to produce a complex product, also called a compound. Sometimes synthesis reactions can result in the formation of more than one product, as we'll see shortly with the process of photosynthesis.

Whether it is one complex product or multiple products, a great way to remember what a synthesis reaction is, is to think of the word 'combination.' Essentially, you are combining reactants to make a desired product. The formula for a synthesis reaction, involving the formation of a complex product, is shown here in Diagram 1. As you can see, substance D combines with substance R to produce a more complex compound, labeled DR.

Diagram 1: Formula for a synthesis reaction

As you can gather from the equation shown in Diagram 1, a synthesis reaction is not a multi-step process. That is, there is no need to be concerned with learning multiple steps for this reaction. In fact, a synthesis reaction is perhaps one of the most widely used reactions; not just in chemistry, but also in unconventional places, such as our kitchens.

Before we practice a few examples, there are two concepts to keep in mind regarding synthesis reactions. First, binary compounds can be produced from a synthesis reaction. A binary compound is a compound that contains only two different elements. For example, the combination of two reactants, iron and oxygen, will produce the binary compound iron oxide, commonly known as rust.

Second, always remember when working with synthesis reactions to balance your equation. As you will see shortly, coefficients, for example numbers, are sometimes required to ensure what is present on the left side of a reaction is equal in amount to what is present on the right side of a reaction. Now that we have addressed these two fundamental concepts, let's apply what we've learned to a few examples.

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Example 1: You are working diligently in a scientific laboratory and decide to mix a solution containing lithium with another solution containing chlorine. What do you predict will be produced from the mixture of these solutions?

Elements under question: lithium (Li) and chlorine (Cl2)

Synthesis equation:

licl unbalanced

Balanced synthesis equation:


From this synthesis reaction, we can see that 2 molecules of lithium chloride (LiCl2) are produced. Because two molecules of chlorine (Cl2) are present as a reactant, this needs to be balanced for the product LiCl (where only one chlorine atom is present). The addition of a coefficient of 2 (in front of LiCl) will ensure all chlorine atoms, on both the reactant and product side, are balanced. However, we cannot stop right there with balancing. A coefficient of 2 must be added to the lithium atom on the reactant side, to ensure all lithium atoms (on both sides of the reaction) are balanced.

Example 2: The process of photosynthesis involves the use of a synthesis reaction. What is produced when carbon dioxide reacts with water during photosynthesis?

Elements under question: carbon dioxide (CO2) and water (H2O)

Synthesis equation (balanced):


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Additional Activities

Synthesis Reactions

Observing a chemical reaction can be a good way to remember how to categorize a reaction based on the various reaction types. In this activity, we're going to conduct an experiment with a single replacement reaction. We're going to react steel, which is an alloy of iron, with oxygen.

Synthesis reactions are also called combination reactions because multiple reactants form a single product. The carbon in steel is added to toughen the steel and won't prevent the reaction of the iron in the steel with oxygen in the atmosphere. Rust is iron oxide, which is the chemical combination of iron and oxygen.

Materials Required:

  • piece of steel wool
  • Bunsen burner
  • matches
  • aluminum foil
  • tongs

Safety Precautions:

Tie back any hair and don't wear baggy sleeves. Keep a glass of water nearby in case any fire needs to be extinguished. Also, always have a responsible adult observing any experiments you may conduct.


  1. Place a large sheet of aluminum foil on a flat surface and put the Bunsen burner on the aluminum foil.
  2. Write down the appearance of the steel wool on a piece of paper and label it "before reaction".
  3. Carefully light the Bunsen burner and wait for a stable flame.
  4. Use the tongs to pick up the steel wool and hold the steel wool in the fire.
  5. Remember the color of the steel wool when it gets hot. Record this observation later when the flame is out and the steel wool is safely back on the surface.
  6. After the steel wool gets red hot take it away from the flame and record the color of it after it cools down.
  7. Place the steel wool safely on the aluminum foil and extinguish the flame.


1. Balance this reaction for mass: {eq}Fe + O_2 \longrightarrow FeO {/eq}.

2. Why do you think the steel wool changed color after it was heated?

3. Describe why this is a synthesis reaction.

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