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Chemical Reactions and Equations – Introduction

A substance can undergo a change by two ways –
(i) Physical changes
This is a change in physical properties such as shape, size, colour and state of a substance.

  • No new substance is formed.
  • A physical change is generally reversible.
    e.g. Boiling of water, condensation of water vapour, melting of ice, dissolution of sugar in water etc.

(ii) Chemical changes
This is a change in which one or more new substances with new properties are formed.

  • Original substance loses their nature and identity.
  • Rearrangement of atoms take place.
  • A chemical change can be reversible or irreversible.
  • A chemical change is also known as chemical reaction.
    e.g. Rusting of iron, souring of milk, burning of fuel, cooking of food, respiration, digestion of food in our body, ripening of fruits etc.

Thus, the process involving a chemical change is called a chemical reaction. The presentation of a chemical change in the equation is known as chemical equation.

Activity – 1


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Characteristics of Chemical Reaction

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