Reactants, Products, and Word Equations

Reactants are the starting substances of a chemical reaction, products are the new substances formed, and a word equation is a way of writing a reaction in words with an arrow that means “forms” or “produces”. In brief: the word equation “hydrogen + oxygen → water” shows that hydrogen and oxygen are the reactants and water is the product.

Word equations are the first step toward writing chemical equations with symbols and formulas. They summarize what is reacting and what is formed, without needing to know formulas. This article explains the parts of a word equation, gives the rules for writing one, lists examples for common reaction types, works through an exercise from a description of an experiment and explains how word equations lead to symbol equations.

What are reactants and products?

A reactant is a substance that is present at the start of a reaction and is changed during it. A product is a substance that is present at the end and was formed in the reaction. The reactants are written on the left of the arrow, and the products on the right.

In the burning of charcoal, carbon and oxygen are the reactants, and carbon dioxide is the product. The reactants are used up, so their amounts decrease, while the amount of product increases.

How do you write a word equation?

1. Identify the starting substances and write their names on the left side.
2. Separate several reactants with a plus sign (+).
3. Write an arrow (→) pointing to the right. Read it as “react to form” or “produces”.
4. Write the names of the products on the right side, separated by plus signs.
5. Use the full name of each substance, such as sodium chloride, not “salt” in general.

The order of the reactants or of the products does not matter, so “iron + sulfur → iron sulfide” and “sulfur + iron → iron sulfide” mean the same.

Word equations for common reactions

Can a word equation show conditions?

The conditions that are needed for the reaction, such as heat, light, a catalyst or electricity, are usually written above or below the arrow. For example: calcium carbonate → (heat) calcium oxide + carbon dioxide. Photosynthesis needs light and chlorophyll: carbon dioxide + water → (light, chlorophyll) glucose + oxygen. A word equation does not show the amounts of the substances, because that requires a balanced symbol equation.

Worked example: writing word equations

Write the word equation for each description and identify the reactants and products.

1. “A piece of iron wool is heated in oxygen and glows, forming a black solid, iron oxide.” Iron + oxygen → iron oxide. Reactants: iron, oxygen. Product: iron oxide.
2. “Copper(II) oxide black powder is added to warm sulfuric acid, and a blue solution forms.” Copper(II) oxide + sulfuric acid → copper(II) sulfate + water. The blue solution contains copper(II) sulfate.
3. “Calcium hydroxide solution (limewater) is bubbled with carbon dioxide and turns cloudy.” Calcium hydroxide + carbon dioxide → calcium carbonate + water. The cloudiness is solid calcium carbonate.
4. “Hydrogen peroxide decomposes in the presence of manganese(IV) oxide, forming water and oxygen.” Hydrogen peroxide → water + oxygen. Manganese(IV) oxide is a catalyst. It speeds up the reaction but is not used up, so it is written above the arrow and not as a reactant.

How do word equations lead to chemical equations?

Replacing each name with a formula gives a skeleton equation. For example, “magnesium + oxygen → magnesium oxide” becomes Mg + O₂ → MgO. The number of atoms of each element must be the same on both sides, so the equation is balanced as 2Mg + O₂ → 2MgO. Balancing is covered in later articles. The word equation helps because it shows the substances first, before the formulas have to be correct.

How did chemists write reactions in the past?

Alchemists used symbols for metals and substances, often linked to planets, such as the Sun for gold and the Moon for silver. In the early 1800s, Jöns Jacob Berzelius introduced the system of letter symbols that is used today, based on the Latin names of elements. His notation made modern chemical formulas and equations possible, with word equations remaining a useful teaching tool.

Which mistakes are common?

Mistake 1: Putting a catalyst among the reactants

A catalyst is not used up, so it is written over the arrow.

Mistake 2: Using an equals sign

Use an arrow, since a reaction goes from reactants to products and the two sides are not the same substances.

Mistake 3: Writing the products first

Reactants always go on the left and products on the right.

Mistake 4: Naming the product incorrectly

Burning magnesium gives magnesium oxide, not “magnesium ash”. Use the chemical name of the substance.