A substance that cannot be broken down into simpler substances is referred to as what?

The correct answer is an element. An element is defined as a pure substance that cannot be broken down into simpler substances by chemical means. This means that elements are the fundamental building blocks of matter, and each element consists of only one type of atom. For instance, elements such as hydrogen, oxygen, and carbon are each made up of only their own kinds of atoms and cannot be decomposed into simpler substances.

In contrast, a compound consists of two or more elements that are chemically combined in fixed proportions, making it possible to break them down into their simpler elemental components via chemical reactions. A mixture is a physical combination of two or more substances where each retains its own properties and can be separated by physical means, such as filtration or evaporation. A colloid is a specific type of mixture where tiny particles of one substance are evenly distributed within another, but the particles do not settle out as in a true solution. Understanding these distinctions helps to clarify why an element is the correct choice when discussing substances that cannot be further broken down.