Reactants and Products

Core Idea

In a chemical reaction, the starting substances are called reactants and the new substances that form are called products. Reactants are transformed into products as atoms rearrange and form new chemical bonds. A chemical equation uses an arrow to show this transformation: reactants are written on the left, products on the right. No atoms are created or destroyed — the same atoms that start in the reactants end up in the products, just arranged differently.

How It's Best Learned

Start with a simple, visible reaction — like vinegar and baking soda producing carbon dioxide gas. Identify what you started with (reactants) and what you ended up with (products). Then practice writing the reaction in words before moving to chemical symbols.

Common Misconceptions

• Atoms are not created or destroyed in a chemical reaction — they are rearranged. The same atoms present in the reactants appear in the products.
• The arrow in a chemical equation does not mean "equals" — it means "produces" or "yields." The substances on each side are different.
• Reactants do not simply disappear — they are converted into products. If a solid seems to vanish, it may have become a gas or dissolved into a liquid.
• Not every change you can see is a chemical reaction — dissolving and melting are physical changes, not chemical reactions.

Explainer

You have learned the difference between physical and chemical changes, and you know how to read chemical formulas. Now it is time to look at chemical reactions more closely and learn the vocabulary scientists use to describe them.

Every chemical reaction has two sides: what you start with and what you end up with. The starting substances are called reactants — these are the ingredients that undergo the chemical change. The new substances that form are called products — these are the results of the reaction. For example, when you light a match, the chemicals on the match head (reactants) react with oxygen in the air to produce heat, light, and new substances like carbon dioxide and water (products).

Scientists describe reactions using chemical equations. A simple word equation for rusting might look like: iron + oxygen → iron oxide. The arrow (→) means "produces" or "yields." Everything on the left side of the arrow is a reactant; everything on the right is a product. The plus sign (+) separates multiple reactants or multiple products. As you learn element symbols and formulas, these word equations get replaced by formula equations, but the structure stays the same.

The most important principle to understand about chemical reactions is that atoms are rearranged, not created or destroyed. Before the reaction, you have a certain collection of atoms arranged as the reactants. After the reaction, you have the exact same collection of atoms, but now they are arranged as the products. If you start with iron atoms and oxygen atoms, you end with iron atoms and oxygen atoms — they have just been reorganized into iron oxide molecules. No atom vanishes, and no new atom appears from nowhere.

This rearrangement happens because chemical bonds break and new bonds form. In the reactants, atoms are bonded one way. During the reaction, those bonds break (which requires energy) and new bonds form between different atoms (which releases energy). The products have different bond arrangements than the reactants, which is why products have different properties. Iron is a shiny, strong metal. Oxygen is an invisible gas. But iron oxide — rust — is a reddish-brown, crumbly solid. The atoms are the same; the arrangement has changed.

Understanding reactants and products gives you the framework for thinking about all chemical reactions. Whether it is baking a cake, running a car engine, or digesting food, every chemical reaction follows the same pattern: reactants transform into products as atoms rearrange.