Answer: Chemical reaction
Step-by-step explanation:
Element is a pure substance which is composed of atoms of similar elements.It can not be decomposed into simpler constituents using chemical reactions.Example: Iron
Compound is a pure substance which is made from atoms of different elements combined together in a fixed ratio by mass.It can be decomposed into simpler constituents using chemical reactions. Example: water
Thus as can be seen from the reaction , the compound water can be separated into its constituent elements hydrogen and oxygen by decomposition reaction which is a chemical reaction.