Question

What is a chemical reaction? Give examples.

Answer 1

Chemical reaction is a process by  which one or more
substances, known as reactants, are converted to one or more different substances,  known as  the products.

Lets assume that X and Y are reactants.
Z is the product

Then , 
X reacts  with Y to form Z

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Examples : Burning fuels , etc .....

Answer 2

Hi friend,
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chemical reactions simply involve exchange of partners in atoms of different elements that are forming compounds.
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Examples;-

1)Cao + H2O ----> Ca(OH)2 
2) 2H2 + O2------> 2H2O
3)H2+ Cl2------>2HCl
4)2FeSO4------>Fe2O3+SO2+SO3
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