If matter is uniform throughout and cannot be separated into other substances by physical processes, but can be decomposed into other substances by chemical processes, it is called a(n)

Compound

Explanation:

A compound is a substance that cannot be broken down by an ordinary physical process but can be split into its component elements by chemical processes such as decomposition, dissociation, displacement.

A compound is made up of two or more different atoms. A particular compound is unique/uniform in its composition and doesn't share its particular composition with another compound. For example, carbon monoxide (CO) is made up of one atom of carbon and one atom of oxygen while carbon dioxide (CO₂) is made up of one atom of carbon and two atoms of oxygen.

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c (diamond),

sio2 (quartz)