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Atoms and Molecules




Bonds and Octet Rules
Chemical Equations
The Mole




Functional Groups
Periodic Table


Practice Quiz I
Practice Quiz II
Practice Quiz III

Chemical Equations

Chemical equations are the sentences in the language of chemistry. Each equation describes a single reaction. For all the millions of different chemical reactions that have been identified, there exists an equal number of chemical equations, and with every discovery of a new chemical reaction, a new chemical equation is born.

A chemical equation offers a great deal of information, such as:

1) The different types of matter involved in the reaction
2) The states of all the different types of matter (solid, liquid, gas)
3) A reflection of the law of conservation of mass, that is, that the same numbers and types of atoms appear on both sides of the arrow in the equation.

The Anatomy of a Chemical Equation

There are several parts to a chemical equation. To understand the different parts of a chemical equation, we will use the reaction that produces water from hydrogen and oxygen:

Parts of chemical equation

Balancing Chemical Equations

If you look at the equation above, where hydrogen gas and oxygen gas combine to produce water, you might notice two things that are kind of odd. First, there are two atoms of oxygen on the reactant side of the equation, but only one atom of oxygen on the product side, which suggests that an atom of oxygen was “lost” somewhere during the reaction. Secondly, the total mass of the two reactants is a lot larger than the total mass of the one product (34 amu of reactants but only 18 amu of product).


According to the equation, almost half of the reactant matter was destroyed during the reaction while producing the product. But this would violate the Law of Conservation of Mass: the total mass of the reactants must equal the total mass of the products. Thus, we have to find a way to make the above reaction agree with the Law of Conservation of Mass, and the way we do this is by balancing the chemical equation.

A balanced chemical equation has an equal number of each type of atom on both sides of the equation. In the case of the reaction that produces water, this means that equal numbers of hydrogen atoms and oxygen atoms need to be on both sides of the equation. Since every atom in the balanced chemical equation has a counterpart on the other side of the equation, the mass of the reaction is always conserved.


To balance an equation, you add matter to both sides until there are equal numbers of each type of atom on both sides of the equation. When adding this matter to the equation, there are some things to consider:

1) Only matter present in the actual reaction can be used to balance the equation. We cannot add a molecule of carbon monoxide to the product side of the equation to balance the oxygen on the reactant side because carbon monoxide is not present in the actual reaction. Only hydrogen, oxygen, and water can be used to balance the equation.

2) Matter can only be added to the side of the equation where it naturally exists. Since oxygen is not produced by the reaction, oxygen cannot be added to the product side to balance the oxygen on the reactant side. Only water can be added on the product side because water is the only product of the reaction.

Let us look at the balancing of our equation.

Here is how to balance our water equation

Here is another example of how to balance a chemical equation, using the reaction of butane and oxygen

As you can see in the above illustrations, a chemical equation is not necessarily the same as a mathematical equation. If you wanted to add a number to one side of a mathematical equation, you would have to add that number to the other side to keep each side of the equation equal to the other. But a chemical equation does not follow this rule, because each side of a chemical equation is not equal to the other—in terms of the number and identities of the atoms appearing on both sides of the arrow—until the equation is balanced. It is through balancing that both sides of a chemical equation become equal to each other with regard to the number and identities of the atoms involved.