If a substance cannot be separated by physical means or chemical means, it is

If a substance cannot be separated by physical means or chemical means, it is an element.

Explanation:

A chemical element is a substance that cannot be broken down into other substances by chemical means and methods. An element consists of one kind of atoms with all the same atomic number, so with the same number of protons in the nucleus. The number of neutrons in the atoms of an element can vary.  

The elements are subdivided into metals, metalloids and non-metals. Most elements are a solid at room temperature. Exceptions include the noble gases that are gaseous, and mercury, which is liquid.