reactant

Imagine you're baking a cake. You need flour, sugar, and eggs—those are your reactants. In chemistry, a reactant is any substance that is consumed or transformed during a chemical reaction. It's what you start with before the change occurs! Reactants combine and rearrange their atoms to create new substances called products. Think of it like building with LEGO bricks: the individual pieces (reactants) are put together to create a finished model (the product).

You'll often hear about reactants when discussing lab experiments or chemical formulas. For example, in photosynthesis, carbon dioxide and water are the reactants that plants use to make sugar and oxygen. Without those starting materials, there would be no cake... and no plant life!

A reactant is a substance that participates in a chemical reaction, undergoing a fundamental change as it transforms into one or more different substances known as products. It is essentially an "ingredient" in a chemical recipe. While we often think of reactions happening only in labs with beakers and test tubes, they are actually happening all the time around us—from the rusting of a car to the way your body breaks down food.

In a chemical equation, reactants are always written on the left side of the arrow (→), which points toward the products to show the direction of the change. For example, in the equation 2H₂ + O₂ → 2H₂O, hydrogen (H₂) and oxygen (O₂) are the reactants that combine to form water. It is important to distinguish reactants from catalysts; while a catalyst helps a reaction happen faster, it isn't "used up" like a reactant is.

The word is used mainly in science, especially chemistry, biology, and related fields. You will see it in textbooks, classroom discussions, lab reports, and scientific articles, where it refers to the materials present before a reaction takes place. Understanding reactants is fundamental to chemistry because they help explain what is changing, what is being used, and how new substances are formed. From the creation of modern plastics to the life-sustaining processes inside every living cell, reactants are at the start of the story.