# How to balance a chemical equation

Balancing a chemical equation is very important in order to calculate the amount of reagents needed and the products in a reaction. After all, it’s an equation, so you must have the same quantities on both sides.

Let’s take the reaction between hydrogen and oxygen, which yields water:

That small number specifies the number of atoms of an element and it can’t be changed. Here, the elements are hydrogen H and oxygen O, but they exist as biatomic gases, which means two atoms of hydrogen make a molecule and the same is for oxygen. The problem here is that we have two atoms of hydrogen and two of oxygen on the left-hand side but two of H and 1 of O on the right. The O in H2O doesn’t have that small “two” because it’s part of a compound, in this case water. To fix this inequality we can only change the number of molecules. In order to have two atoms of oxygen on the right-hand side, we will write 2 next to H2O:

This means that we now have four hydrogens and two oxygens, so we need two more atoms of H on the left, which means one more molecule of H2.

Now we can say that the equation is balanced.

Let’s now balance this equation:

We have one atom of carbon on both sides but 2 vs 1 of oxygen. What we’re going to do is write a 2 next to CO, so we will have 2 atoms of oxygen:

We now need to fix the carbon, since we have one vs two:

And that’s it!

Now try to balance this chemical equation:

This is the reaction between carbon dioxide and water, and gives carbonic acid. This is sparkling water! And as you know, after some time the gas escapes leaving water only. This is why I’ve used two arrows: carbon dioxide reacts with water to yield carbonic acid but carbonic acid can decompose to give the reagents back. The procedure doesn’t change though.