A chemical equation is a theoretical or written representation of what happens during a chemical reaction. The law of conservation of mass states that no atoms can be created or destroyed in a chemical reaction, so the number of atoms that are present in the reactants has to balance the number of atoms that are present in the products.

Write down your given equation. For this example, we will use:

C3H8 + O2 --> H2O + CO2

Write down the number of atoms that you have on each side of the equation. Look at the subscripts next to each atom to find the number of atoms in the equation.

  • Left side: 3 carbon, 8 hydrogen and 2 oxygen
  • Right side: 1 carbon, 2 hydrogen and 3 oxygen

As we can see, the above Equation is not Balanced. To balance them, we can follow the below steps:

Always leave hydrogen and oxygen for last. This means that you will need to balance the carbon atoms first.

Add a coefficient to the single carbon atom on the right of the equation to balance it with the 3 carbon atoms on the left of the equation.

C3H8 + O2 --> H2O + 3CO2

  • The coefficient 3 in front of carbon on the right side indicates 3 carbon atoms just as the subscript 3 on the left side indicates 3 carbon atoms.
  • In a chemical equation, you can change coefficients, but you should never alter the subscripts.

Balance the hydrogen atoms next. You have 8 on the left side, so you'll need 8 on the right side.

C3H8 + O2 --> 4H2O + 3CO2

  • On the right side, we added a 4 as the coefficient because the subscript showed that we already had 2 hydrogen atoms.
  • When you multiply the coefficient 4 times the subscript 2, you end up with 8.

Finish by balancing the oxygen atoms.

  • Because we've added coefficients to the molecules on the right side of the equation, the number of oxygen atoms has changed. We now have 4 oxygen atoms in the water molecule and 6 oxygen atoms in the carbon dioxide molecule. That makes a total of 10 oxygen atoms.
  • Add a coefficient of 5 to the oxygen molecule on the left side of the equation. You now have 10 oxygen molecules on each side.

    C3H8 + 5O2 --> 4H2O + 3CO2.

  • The carbon, hydrogen and oxygen atoms are balanced. Your equation is complete.