When is an equation called balanced / unbalanced?

An equation is said to be balanced when there are equal number of molecules for each element on both sides of the equations.

When approaching a chemical equation, it is important that you understand the difference between coefficients and subscripts. The coefficient is placed in front of a molecule, while the subscript follows certain atoms.

In a molecule, the coefficient denotes the amount of that molecule present. The subscript of an atom indicates the amount of that atom in the molecule.

If there is not a subscript present on an element, you can infer there is only one atom of that element.

Adding a coefficient in front of a molecule multiplies all atoms within that molecule by the number of the coefficient. If an atom has a subscript, the coefficient and the subscript multiply to yield the total amount of that atom in the molecule.

The coefficient is the part that can be changed and added when balancing an equation. Changing the coefficient changes the total number of that molecule. The subscript, however, cannot be changed. Altering a subscript would change the molecule itself.

Example:

2Na + Cl2 -> 2NaCl

In the above equation we have, 2 Sodium (Na) and 2 Chlorine (Cl) on both L.H.S and R.H.S, Hence the equation is said to be balanced. Generally, an equation is said to be unbalanced when the number of molecules for each element are not equal on both sides.

Example:

H2 + O2 -> H2O

Considering the equation above, we have 2 hydrogens (H) and 2 oxygen (O) on the L.H.S and 2 hydrogens (H) and only 1 oxygen (O) on the R.H.S. Hence with the unequal number of oxygen molecules the chemical equation is said to be unbalanced.

How to balance an unbalanced chemical equation?

Let us learn here how to balance the above-unbalanced equation with step by step procedure.

Given,

H2 + O2 -> H2O

To balance this the unbalanced oxygen molecules, we add 2 in front of the product on R.H.S. We get,

H2 + O2 -> (2) H2O

Now, we have 2 Oxygen's but 4 Hydrogen's on the R.H.S. In order to balance the same, we add an another 2 in front of Hydrogen on the L.H.S

2 H2 + O2 -> 2 H2O

Now, the equation is balanced with 4 Hydrogen's (H) and 4 Oxygen's on both sides.