pure
That would be an Element.
If the above didn't answer your question, a substance that can be broken down into simpler substances is called a compound. Even the simplest compounds can be broken down into individual elements.
Atom of carbon?
A substance that cannot be broken down into simpler parts is called an element. Elements are the basic building blocks of matter and consist of only one type of atom. They are represented on the periodic table and include substances like hydrogen, oxygen, and gold. Each element retains its unique properties and cannot be chemically transformed into a simpler substance.
That process is called "thermal decomposition," where a substance is broken down into simpler compounds or elements through the application of heat.
A chemical element. All the known chemical elements are listed in the 'periodic table'.
This is called molecule.
This kind of reaction is called "decomposition".
compounds
elements
an element
The process of changing food into a simpler substance is called COMBUSTION