How to balance a chemical reaction?
Balanced chemical equation is an equation in which the number of atoms for each element on the reactant side and product side is equal.
Let us see the steps to be followed for writing a balanced chemical equation
Fe + H2O 
Fe3O4 + H2
Step 1- To balance a chemical equation, draw boxes around each formula.
Step 2- Now write down the number of atoms of different elements present in the unbalanced equation i.e., here we list out the number of atoms of Iron (Fe), Hydrogen (H), and Oxygen (O).
| Element | Number of atoms in reactants (LHS) | Number of atoms in products (RHS) |
| Fe | 1 | 3 |
| H | 2 | 2 |
| O | 1 | 4 |
Step 3- For balancing a chemical equation, it is more convenient if we start balancing with the compound that contains the maximum number of atoms. It can be a reactant or a product. If we consider this criterion, we take Fe3O4 and the element oxygen. There are four oxygen atoms on the product side and only one on the reactant side. To balance the oxygen atoms on both sides-
| Atoms of oxygen | In reactants | In products |
| i. Initial | 1 (in H2O) | 4 ( in Fe3O4) |
| ii. To balance | 1×4 | 4 |
You should keep in mind that while making the number of atoms equal, we cannot alter the formula of the compound or elements involved in the reaction.
The partially balanced equation becomes-
Step 4- To proceed further, we need to balance Fe and H atoms. Balance the hydrogen atoms in the partly balanced equation.
Here, to equalise the number of hydrogen atoms, we have to make the number of molecules of hydrogen as four on the product side.
| Atoms of hydrogen | In reactants | In products |
| i. Initial | 8 (in 4 H2O) | 2 (in H2) |
| ii. To balance | 8 | 2×4 |
Now the equation would be-
Step 5- Now that we have balanced both oxygen and hydrogen atoms, the only element left to be balanced is Iron (Fe).
| Atoms of Iron | In reactants | In products |
| i. Initial | 1 (in Fe) | 3 (in Fe3O4) |
| ii. To balance | 1×3 | 3 |
To make the iron atoms equal on both sides, we take three atoms of Fe on the reactant side.
Step 6- At the end make sure that the equation is balanced. This you can check by counting the atoms of each element on both sides of the equation is equal.
3Fe + 4H2O Fe3O4 + 4H2
you can see that
- The number of atoms of Iron (Fe) is 3 on both sides
- The number of atoms of oxygen (O) is 4 on both sides
- And that of hydrogen is 8 on both sides
Thus, we get a balanced chemical equation. This method is also known as hit- and -trial method as we make trials to balance the equation.
Step7- We have to write the symbols of the physical states of each element in the equation. To make the equation more informative, the physical states of the reactant and product are mentioned along with their chemical formulae.
| Physical State of the element | Representation |
| Solid | s |
| Liquid | l |
| Gaseous | g |
| Aqueous (a solution in water) | aq |
The balanced equation is-
3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g)
Here you can see that the symbol (g) is used for H2O. This indicates that in this reaction water is used in the form of steam. Unless it is necessary to specify the physical states of the elements, they are not included in a chemical equation. In some cases, to show the reaction condition like temperature, pressure, catalyst, etc.., they are indicated above and/or below the arrow in the equation.
Solved Examples
Example 1: H2 + Cl2 HCl
Step 1- Write the number of atoms of each element in the unbalanced equation.
| Elements | Number of atoms in reactants (LHS) | Number of atoms in products (RHS) |
| H | 2 | 1 |
| Cl | 2 | 1 |
Step 2- Now, let us balance the hydrogen atom.
| Atoms of hydrogen | In reactants | In products |
| i. Initial | 2 (in H2) | 1 (in HCl) |
| ii. To balance | 2 | 1×2=2 |
H2 + Cl2 2HCl
Step 3- When hydrogen got balanced, now we have to check whether the number of atoms of chlorine is balanced.
| Atoms of Chlorine | In reactants | In products (after hydrogen atom got balanced) |
| i. Initial | 2 (in Cl2) | 2 (in HCl) |
| ii. To balance | 2 | 2 |
Thus, we get the balanced equation as
H2 + Cl2 2HCl
Example 2: Na + H2O NaOH + H2
Step 1- Write the number of atoms of each element in the unbalanced equation.
| Elements | Number of atoms in reactants (LHS) | Number of atoms in products (RHS) |
| Na | 1 | 1 |
| H | 2 | 3 |
| O | 1 | 1 |
Step 2- To balance the number of hydrogen atoms, multiply 2 with NaOH in the product.
| Elements | Number of atoms in reactants (LHS) | Number of atoms in products (RHS) |
| Na | 1 | 2 |
| H | 2 | 4 |
| O | 1 | 2 |
Na + H2O 2NaOH + H2
Step 3- To balance the number of sodium atoms, we multiply the Na in the reactants by 2.
| Elements | Number of atoms in reactants (LHS) | Number of atoms in products (RHS) |
| Na | 2 | 2 |
| H | 2 | 4 |
| O | 1 | 2 |
2Na + H2O 2NaOH + H2
Step 4- As you can see, the number of hydrogen atoms and oxygen atoms is not balanced. We multiply H2O by 2.
| Elements | Number of atoms in reactants (LHS) | Number of atoms in products (RHS) |
| Na | 2 | 2 |
| H | 4 | 4 |
| O | 2 | 2 |
2Na + 2H2O 2NaOH + H2
Thus, we arrived at the balanced chemical equation.