# Substance --- A **substance** in [[chemistry]] is a pure form of [[matter]]. It contains *only* one type of [[element]] or one type of [[molecule]]. This is as opposed to a [[mixture]], which is made up of more than one type of element or molecule. ## Chemical Compounds A **chemical compound** is substance when two or more [[element|elements]] that have been [[chemical bond|chemically bound]] together, and have properties different from those of its component elements. Compounds are similar in definition to [[molecule|molecules]], but the definition of molecule is broader and includes atoms of *one* or more elements bound together. So, for example, O$_2$ wouldn't count as compound. ___