calcium oxide as CaO, water as H₂O and the compound formed by the reaction of these two compounds is calcium hydroxide Ca(OH) ₂.

Now the reaction of calcium oxide with water can be written as:
CaO + H₂O ---> Ca (OH)₂ ............................ (2)

In the above chemical equation, count the number of atoms of each element on left side and right side of arrow.

• Is the number of atoms of each element are equal on both sides?

Observe the following reactions and their chemical equations. Zinc metal reacts with dilute HCl to yield ZnCl₂ and liberates Hydrogen gas.
Zn + HCl ---> ZnCl₂ + H₂ ............................ (3)

Sodium sulphate reacts with barium chloride to give white precipitate, barium sulphate and sodium chloride.
Na₂SO₄ + BaCl₂---> BaSO₄ + NaCl ............................. (4)

• Do the atoms of each element on left side equal to the atoms of the element on the right side of the equation?

According to the law of conservation of mass, the total mass of the products formed in chemical reaction must be equal to the total mass of reactants consumed. You know that an atom is the smallest particle of an element that takes part in a chemical reaction. It is the atom which accounts for the mass of any substance. The number of atoms of each element before and after reaction must be the same.

All the chemical equations must be balanced, because atoms are neither created nor destroyed in chemical reactions. A chemical equation in which the number of atoms of different elements on the reactant side (left side) are same as those on product side (right side) is called a balanced reaction.

Formula units: Balancing a chemical equation involves finding out how many formula u