Question

why decomposition reaction is called the opp of combination reaction

Answer 1

Combination reaction :
A reaction in which two or more elements combine to form compound.
Deompositon reaction:
A reaction in which a compound breaks down into two or more elements.
As in combination reaction the element combines and in decomposition reaction the compound breaks down that's why it is said that the decomposition reaction is the opposite of the combination reaction. For example :
2H2+O2 --> 2H2O
(Combination reaction)
2H2O--> 2H2+O2
(Decomposition reaction)
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