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)

Mass is the amount of matter in an object.  weight is the measurement of the pull of gravity on an object.  say for example, you travel to the moon which has a different gravitational field compared to the gravitational field of earth. (gravitational fields: earth = 1, moon = 0.165)  now when you are on the moon, the pull of gravity on you is different than on the earth. and as weight is the measurement of the pull of gravity on an object, that means yes, your weight can and will change on the moon.  however, on the moon, on earth or even on jupiter, the amount of mass inside of you remains the same. meaning your physical mass remains constant.  in conclusion it is possible for your weight to change as your mass remains constant.