A chemical change is the one in which formation of one or more new substances takes place. The new substance formed has different chemical composition and chemical properties from that of the starting material. Also the original material cannot be recovered easily. In simple terms, most of the chemical changes are irreversible.

For example (1)when lime is added to water, the temperature of water increases and water almost starts boiling. A substance called slaked lime is produced during this change. Hence, it is a chemical change. The following chemical equation can be used to represent the chemical change:

CaO + H 2 O → Ca (OH) 2 + Energy

(2) Burning of magnesium is a chemical change as it reacts with oxygen to form a new substance magnesium oxide which is irreversible by any physical methods.

Magnesium + Oxygen → Magnesiumoxide

Mg + O2 → MgO2

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