A chemical reaction takes places when one molecule or set of molecules is transformed into another type of molecule or set of molecules. In order to describe this process, scientists make use of chemical equations.

A chemical equation is a symbolic representation of the chemical reaction. It is written using element names as taken from the periodic table and indicates reactants and products.

The following equation shows the chemical equation for a combustion reaction:

On the left of the arrow are the reactants for this reaction, while on the right are the products. The arrow indicates the direction that the reaction proceeds.

You can see that the reaction takes methane and oxygen in order to yield carbon dioxide and water. You can also see that there is a coefficient next to both the oxygen and the water. Why is this?

The Law of Conversation of Mass states that atoms are neither created nor destroyed during a chemical reaction. As a result, the total number of atoms of each type must always be the same on either side of the chemical equation.

Since the reactants and the products do not change, often the only way to make sure the same number of atoms are on either side of the arrow involves adding coefficients to the individual compounds involved.

Take a look at the combustion reaction shown above again. This time, it does not have any coefficients:

On the reactants side there is one atom of carbon, four atoms of hydrogen and two atoms of oxygen.

On the products side there is one atom of carbon, two atoms of hydrogen and three atoms of oxygen.

The Law of Conservation of Mass has been broken! There are not equal number of hydrogen or oxygen atoms on either side. The reactant side has four atoms of hydrogen while the product side only has two.

This means coefficients are required to ensure that there are equal numbers of atoms on either side. In this case, by adding a two as a coefficient to water, you end with four atoms of hydrogen on either side, which is exactly what is needed. So, hydrogen is conserved.

Lastly, the number of oxygen atoms needs to be balanced. After adding the coefficient of two to the water, there are four atoms of oxygen on the product side and only two on the reactant side. By adding a coefficient of two to the O₂ reactant, this imbalance can be fixed. With this, there are now four atoms of oxygen on either side.

Here is the balanced version of the equation:

Though balancing equations always takes some guess work and some trial and error, there are some basic rules you can follow.

1. Tally the number of atoms for each element on each side. 
2. Choose one element to begin with. It is usually best to begin by choosing the element that there are the most of. Balance that one element by adding coefficients.
3. Recalculate the total number of atoms for each element after the addition of coefficients.
4. Balance the next element by adding coefficients.
5. Continue this process until all elements are balanced.
6. Deal with polyatomic ions as one unit.
7. If some compounds are charged make sure that the charges are also balanced.
8. Always finish by checking your work.

For example, balance the equation for the overall photosynthesis reaction:

1. Tally the number of atom of each element on each side.

2. Choose an element to begin with. It is usually a good idea to begin with the element that has the most atoms. In this case that is the 12 hydrogen atoms in the product side. The reactant side must be changed so that there are 12 hydrogen atoms on that side as well. To do this, you can add a coefficient of 6 in front of the water since water contains two hydrogens.

3. Now, recalculate the total number of atoms.

4. Next, you can move onto the carbon since that is clearly still not balanced. To do this, adding a coefficient of 6 in front of the carbon dioxide will yield 6 carbon atoms on the reactant side.

5. Now, recalculate the number of atoms on each side.

6. Now, the only element left to balance is oxygen. There are 6 oxygen atoms in the first product. Then that would leave 12 atoms. If a coefficient of 6 is added in front of the O₂, this would be 12 oxygen atoms.

7. To make sure this is correct, recalculate the number of atoms of each element on either side.

Now, you have a balanced equation for the overall reaction of photosynthesis.