Which term describes a substance that cannot be chemically broken down?

The term that describes a substance that cannot be chemically broken down is "element." An element is a pure substance that consists of only one type of atom. Elements are the fundamental building blocks of matter and are listed on the periodic table. They cannot be broken down into simpler substances by chemical means.

In contrast, a compound consists of two or more different elements that are chemically combined in fixed proportions and can be broken down into its constituent elements through chemical reactions. A mixture is a combination of two or more substances where each retains its own properties and can be separated by physical methods. A solution is a specific type of mixture where one substance is dissolved in another, typically a solid in a liquid, but like mixtures, the components can be separated physically. Therefore, the unique characteristic of elements being indivisible by chemical means distinguishes them from these other categories.