Reactant
Also known as: reagent
A starting substance that is consumed and chemically transformed during a chemical reaction, appearing on the left side of a chemical equation before the reaction arrow. Identifying reactants is the first step in understanding any reaction.
This concept is covered in depth in our complete chemistry definitions reference, with worked examples, practice problems, and common mistakes.
💡 Intuition
What you start with — the ingredients that get used up to make something new.
Core Idea
Reactants appear on the left side of a chemical equation and are used up in the reaction.
Formal View
🔬 Example
🎯 Why It Matters
Identifying reactants is the first step in understanding any reaction. In industry, knowing which reactants to use and in what amounts determines manufacturing cost, safety, and efficiency of chemical production.
⚠️ Common Confusion
Catalysts help reactions but aren't reactants—they're not consumed.
How to Use Reactant
When this concept appears in chemistry, it usually controls how you interpret a representation, a quantity, or a change in a system. Students make faster progress when they can explain what reactant tells them before reaching for an equation or memorized phrase.
A strong self-check is to say what reactant does, what it does not do, and which nearby idea it is easiest to confuse with. That kind of explanation makes later calculations, lab reasoning, and compare pages much more reliable.
💭 Hint When Stuck
When identifying reactants in a problem, look at the left side of the arrow. First read the equation and list everything before the \to symbol. Then check that each substance is actually consumed (not a catalyst or solvent). Finally, note the coefficients — they tell you the mole ratios needed.
Related Concepts
Prerequisites
Next Steps
How Reactant Connects to Other Ideas
To understand reactant, you should first be comfortable with chemical reaction. Once you have a solid grasp of reactant, you can move on to product and limiting reactant.
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Want the Full Guide?
This concept is explained step by step in our complete guide:
Chemistry Terms and Definitions: Product, Reactant, Solution, Base, Molecule →Frequently Asked Questions
What is Reactant in Chemistry?
A starting substance that is consumed and chemically transformed during a chemical reaction, appearing on the left side of a chemical equation before the reaction arrow.
Why is Reactant important?
Identifying reactants is the first step in understanding any reaction. In industry, knowing which reactants to use and in what amounts determines manufacturing cost, safety, and efficiency of chemical production.
What do students usually get wrong about Reactant?
Catalysts help reactions but aren't reactants—they're not consumed.
What should I learn before Reactant?
Before studying Reactant, you should understand: chemical reaction.