Asubstance that can be separated into two or more substances only by a chemical change is a

Explanation:

Compounds are pure substances formed by two or more different kind of atoms.

For example, H₂O, and CaCO₃ are compounds.

Elements can not be separated into other substances by a chemical change. The process to change an element into other substance (different element) is a nuclear reaction (transmutation).

Mixtures are not pure substances and can be separated into other substances by physical changes: for example, a brine can be separated into salt and water by evaporation.

- decrease the number of plants and animals living in the water.