Final answer:
Copper cannot be decomposed by a chemical change.
Step-by-step explanation:
Compounds can be broken down by chemical changes into elements or other compounds. However, elements cannot be broken down further by chemical changes.
Out of the substances given, mercury (II) oxide, potassium chlorate, and water are all compounds and can be decomposed by chemical changes.
Copper, on the other hand, is an element and cannot be decomposed by chemical changes. It remains as copper even when subjected to chemical reactions.
Therefore, the substance that CANNOT be decomposed by a chemical change is copper.