Alberta Boilers Safety Association (ABSA) Practice Test

The correct answer is an element. An element is defined as a pure substance that consists of only one type of atom and cannot be broken down into simpler substances through chemical reactions. Each element has its own unique properties and is represented by a chemical symbol on the periodic table.

Elements serve as the fundamental building blocks of matter, and they combine in various ways to form compounds and mixtures. While compounds are composed of two or more different elements chemically bonded together, mixtures consist of two or more substances that are not chemically combined.

The other options do not fit the definition of a substance that cannot be broken down into other substances by chemical means. Compounds can be separated into their constituent elements through chemical reactions, and mixtures can easily be separated into their components by physical means. Electrons, while fundamental parts of atoms, are not substances themselves but rather subatomic particles.