Question

. Why is decomposition reactions called opposite of combination reactions? Write equations for these reactions.

Answer 1

Reaction in which two or more reactants combine together to form one product is called combination reaction
CaO+H20=Ca(OH)2

Reaction in which when reacted is divided or decomposed into two or more products is called decomposition reaction
CaCO3=CaO+CO2

By this definition and example you can observe that combination and decomposition reaction are opposite to each other

Hope it helps

Answer 2

decomposition reaction is breaking down of a compound
CaCo3 ----->CaO + CO2
while combination reaction is the reaction in which 2 or more elements react to form product
C + O2 ----> CO2