Asubstance which can be decomposed in two or more simpler substances by chemical action is called a(n)

Answer; 
A compound 
A substance which can be decomposed in two or more simpler substances by chemical action is called a compound. 

Explanation;
A compound is a substance that is made from the atoms of two or more different elements that are chemically combined or bonded.
It is a pure substance that can be decomposed into simpler substances by chemical reactions, can not be separated into 2 or more substances by physical means.
Each compound has a formula showing which elements are present in the compound, and conisists of two or more elements in a fixed ratio.

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