What Is a Chemical Equation?

Woman writing a chemical equation on a chalkboard
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A chemical equation is something you will encounter every day in chemistry. It's a written representation, using numbers and symbols, of the process that occurs during a chemical reaction.

How to Write a Chemical Equation

A chemical equation is written with the reactants on the left side of an arrow and the products of the chemical reaction on the right. The head of the arrow typically points toward the right or the product side of the equation, although some equations may indicate equilibrium with the reaction proceeding in both directions simultaneously.

The elements in an equation are denoted using their symbols. Coefficients next to the symbols indicate the stoichiometric numbers. Subscripts are used to indicate the number of atoms of an element present in a chemical species.

An example of a chemical equation may be seen in the combustion of methane:

CH4 + 2 O2 → CO2 + 2 H2O

Participants in the Chemical Reaction: Element Symbols

You'll need to know the symbols for the elements to understand what is taking place in a chemical reaction. In this reaction, C is carbon, H is hydrogen, and O is oxygen.

Left Side of the Equation: Reactants

The reactants in this chemical reaction are methane and oxygen: CH4 and O2.

Right Side of the Equation: Products

The products of this reaction are carbon dioxide and water: CO2 and H2O.

Direction of Reaction: Arrow

It is the convention to place the reactants on the left side of a chemical equation and the products on the right side. The arrow between the reactants and products should point from left to right or, if the reaction is proceeding both ways, point in both directions (this is common). If your arrow points from right to left, it's a good idea to re-write the equation the conventional way.

Balancing Mass and Charge

Chemical equations may be either unbalanced or balanced. An unbalanced equation lists the reactants and products, but not the ratio between them. A balanced chemical equation has the same number and types of atoms on both sides of the arrow. If ions are present, the sum of the positive and negative charges on both sides of the arrow is also the same.

Indicating States of Matter

It's common to indicate the state of matter in a chemical equation by including parentheses and an abbreviation right after a chemical formula. This can be seen in the following equation:

2 H2(g) + O2(g) → 2 H2O(l)

Hydrogen and oxygen are indicated by (g), which means they are gases. Water is marked (l), which means it is a liquid. Another symbol you may see is (aq), which means the chemical species is in water—or an aqueous solution. The (aq) symbol is a sort of shorthand notation for aqueous solutions so that water doesn't have to ​be included in the equation. It's particularly common when ions are present in a solution.