What is a reactant in simple terms?

A reactant is a substance you start with in a chemical reaction. It is used up or changed as it reacts to form new substances called products.

How can I tell which chemicals are reactants in an equation?

In a chemical equation, the reactants are written on the left side of the arrow. Everything to the left of the arrow is a reactant, and everything to the right is a product.

Do reactants always get used up completely?

Not always. Sometimes one reactant is left over if there is more of it than needed. However, any substance that actually takes part in the reaction and changes is still called a reactant.

Are reactants and ingredients the same thing?

They are similar ideas. Ingredients are the starting materials in cooking, and reactants are the starting materials in a chemical reaction. The big difference is that reactants are defined in terms of chemical changes, not just mixing.

What Is a Reactant? Definition and Examples

Kid-friendly explanation

Imagine making a sandwich. The bread, cheese, and lettuce you start with are like reactants. When you put them together, you get a new thing: a sandwich. In chemistry, reactants are the starting materials that mix or change to make something new.

Worked examples

Example 1 (younger grades): Baking soda and vinegar

When baking soda is mixed with vinegar, bubbles of gas form and the mixture changes. In this reaction, baking soda is a reactant and vinegar is a reactant. They are the starting substances that interact and get used up as they form new substances, including a gas called carbon dioxide.

Example 2 (older grades): Burning methane gas

Consider the chemical reaction: CH₄ + 2O₂ → CO₂ + 2H₂O.

On the left side, CH₄ (methane) and O₂ (oxygen) are the reactants. They are the substances that start the reaction. On the right side, CO₂ (carbon dioxide) and H₂O (water) are the products, the new substances formed when the reactants chemically change.

Going deeper

In a chemical equation, reactants are always written on the left side of the arrow. Products, the new substances formed, are written on the right side. The arrow can be read as "react to form" or "change into."

Reactants are made of atoms, which are tiny particles. During a chemical reaction, the atoms in the reactants are rearranged to form products, but the total number of each type of atom stays the same. This idea is part of the law of conservation of mass.

In more advanced chemistry, you may count how many moles of each reactant you have and use ideas like molarity to describe how concentrated a reactant is in a solution. This helps predict how fast a reaction will happen and how much product can be made.

Common mistakes

Mistake: Thinking reactants and products are the same thing because they are all in the equation.
Correction: Reactants are only the starting substances on the left side of the arrow; products are the new substances on the right side.
Mistake: Believing reactants disappear completely during a reaction.
Correction: Reactants are used up as reactants, but their atoms do not vanish; they are rearranged into products, so the total mass stays the same.
Mistake: Calling anything present in the container a reactant, even if it does not take part in the reaction.
Correction: A reactant must actually change or be used up in the reaction; substances that do not change are not reactants.

Everyday connection

Reactants show up in everyday life whenever something cooks, rusts, or burns. When bread is toasted, substances in the bread are reactants that change into the browned, toasty-smelling compounds on the surface. When a bike chain rusts, iron in the metal and oxygen in the air are reactants that combine to form rust.

Related terms to explore

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What is a reactant?