What is a substance that cannot be broken down into simpler substances called?

A substance that cannot be broken down into simpler substances is called an element. Elements are the basic building blocks of matter and consist of only one type of atom. They cannot be decomposed into smaller components through chemical reactions. Each element has unique properties and is represented by a symbol on the periodic table.

For example, hydrogen (H) and oxygen (O) are both elements; they exist as pure substances and cannot be separated into simpler chemical substances.

In contrast, compounds are substances formed from two or more elements that are chemically bonded together in fixed proportions, and can be broken down into their constituent elements. Molecules refer to two or more atoms bonded together, which can be comprised of either the same element (like O2) or different elements (like H2O). Mixtures are combinations of two or more substances that are not chemically bonded and can be separated through physical means.