A chemist is studying an unknown substance he can break it into its elemental only by using chemical means. Based on this observation, how should the unknown substance be classified

Compound

Explanation:

We want to know if it's a compound or a mixture.

An example of a mixture is salt water: you can heat it hot enough to boil off the water, leaving only the salt. This is a physical change, which is how you know it's a mixture.

Something like gold is a compound: if you heat it, or hit it, you'll still only have gold. You can only break it down by chemical means, which is how you know it's a compound.