Asubstance that cannot be separated into simpler substances by physical or chemical means is a

Pure Substance- Element

Explanation:

Pure Substance VS. Mixture:
A pure Substance CANNOT be separated by physical changes, but a Mixture can.

PURE SUBSTANCE: Element VS. Compound:
An Element is one simple atom while a compound is two or more.
Example: Al (Aluminum) is an Element. CaCl (Calcium Chloride) is a compound.
You can split a compound with chemical changes because if you were to take the example, it would be a Calcium atom and a Chlorine atom.
You CANNOT split a single atom (Element) unless you want to make an atomic bomb which takes a lot of energy and you don’t want to destroy the whole world.

MIXTURE: Homogeneous VS. Heterogeneous:

Homo- mixed; cannot differentiate pieces
Hetero- can differentiate pieces
EX: homo- ketchup (can you see the difference in the vinegar and tomato paste? No)
Hetero- chicken noodle soup (there’s broth, chicken, peas, carrots, and other things)

It's not the same dearthe first one is in grams per mole while the other is not