Which substance cannot be reduced to a simpler substance by chemical means?

An element is a pure chemical substance that consists of only one type of atom and cannot be broken down into simpler substances by chemical means. Elements serve as the foundational building blocks for all matter, and they retain their identity even when subjected to various chemical reactions. Since elements themselves are the simplest forms of matter, any attempt to reduce them chemically will not yield any simpler form; they will remain unchanged.

In contrast, compounds are substances formed when two or more elements chemically bond together, and they can be broken down into their constituent elements through chemical reactions. Mixtures consist of two or more substances that are physically combined, and they can be separated into their individual components without chemical processes. Solutions, as a type of mixture, involve one substance dissolved in another and can also be separated without breaking chemical bonds. Therefore, these other forms of matter can indeed be reduced or separated into simpler components through various chemical or physical methods, which is not the case with elements.