What is a Reactant in Chemistry?

Core Concepts

In this tutorial, you will learn what a reactant is and where to find a one in a chemical equation. You will also learn the difference between a reactant and a reagent, and be introduced to many examples from relevant chemistry.

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What is a Reactant?

The term reactant is used to help describe a chemical equation. A reactant is a chemical substance that is present at the start of a chemical reaction, and is changed as the chemical reaction proceeds. They could also be referred to as terms like starting materials or reagents.

You could also describe a reactant, as something that is present at the beginning of a chemical reaction, and then partially or completely consumed to make the products.

Reactants are found to the left of a chemical equation, before the arrow. For example, in the first equation A and B are reactants and for the second AB and CD are the reactants.

A + BC


This rule applies to these above examples, but also to a vast assortment of other types of reactions. As another example, plants use carbon dioxide and water to make glucose, which is essential to making energy for life.

File:Plant growing towards the sun.jpg

CO2 + H2O C6H12O6 + O2

The materials the plant is using (and changing) are CO2 and H2O. These are our reactants.

Equilibrium Arrows: Where are the reactants?

In reactions that can proceed both backwards and forwards, equilibrium arrows are used to show that the chemistry can run in both directions. In this case, reactants are found on both the left and the right of the chemical equation.

  1. A + BC

2. A + BC

3. C A + B

Equation 1 is telling us that equation 2 and 3 are both able to occur. In equation 1, A and B, but also C are reactants.

Reactants vs. Reagents

Reactants are consumed when a chemical reaction takes place, and a reagent does not have to be consumed. An example of a reagent is a catalyst, which is a material that helps to speed up the reaction, but is unchanged throughout the process. Catalysts can be small molecules, or large complex structures such as enzymes.

In organic chemistry, there are many commonly used reagents, an example is an organometallic reagent like Me-MgBr (pictured below). This is called a Grignard reagent. Since Me-MgBr it is altered in the reaction, it is also considered a reactant.

File:Methylmagnesium bromide coupling reaction small.jpg
Grignard reagent Me-Mg-Br combined with a second reactant, Me-Cl, forms a new molecule.

Note: Solvents, although present when many reactions take place, are not considered reactants or reagents.

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