October 7, 2025

What Is The Difference Between Reactants And Products

Q: Does the mass of the reactants always equal the mass of the products?

Yes, according to the Law of Conservation of Mass, the total mass of the reactants in a closed system is always equal to the total mass of the products.

Q: Can a substance be a reactant in one reaction and a product in another?

Absolutely. For example, water (H₂O) is a product of hydrogen combustion, but it can also be a reactant in the process of electrolysis, where it is broken down into hydrogen and oxygen.

Q: What is a limiting reactant?

A limiting reactant (or limiting reagent) is the reactant that is completely consumed first in a chemical reaction. It determines the maximum amount of product that can be formed.

Q: What if a reactant is left over after a reaction is complete?

Any reactant that is not completely used up when the limiting reactant is gone is called an excess reactant. Some amount of it will remain in the reaction mixture along with the products.

Learn the fundamental distinction between reactants, the starting materials in a chemical reaction, and products, the substances that are formed.

Have More Questions →

Defining Reactants and Products

In chemistry, reactants are the starting substances that are consumed during a chemical reaction, while products are the new substances that are formed as a result of the reaction. Think of reactants as the ingredients and products as the finished dish.

Section 2: Role in a Chemical Equation

In a chemical equation, reactants are always written on the left side of the arrow (→), and products are written on the right. The arrow signifies the transformation, pointing from what is used up (reactants) to what is created (products). Atoms are rearranged from the reactants to form the products, but no atoms are created or destroyed.

Section 3: A Practical Example

Consider the formation of water from hydrogen and oxygen gas. The chemical equation is 2H₂ + O₂ → 2H₂O. In this reaction, hydrogen gas (H₂) and oxygen gas (O₂) are the reactants. They are the initial ingredients that combine and react. The product is water (H₂O), the new substance formed.

Section 4: Why This Distinction Matters

Understanding the difference between reactants and products is essential for balancing chemical equations, predicting the outcomes of reactions, and calculating reaction yields. This core concept is the foundation for stoichiometry, which allows chemists to determine the specific amounts of substances involved in a chemical process.

Frequently Asked Questions

Does the mass of the reactants always equal the mass of the products?

Can a substance be a reactant in one reaction and a product in another?

What is a limiting reactant?

What if a reactant is left over after a reaction is complete?