Classification of Matter
How Are Pure Substances Distinguished from Mixtures?
Pure Substances
Matter falls into three classes. These are elements, compounds, and mixtures. An
elementglossary term (opens in a new window) is a form of matter that consists of one kind of atom. A
compoundglossary term (opens in a new window) consists of the atoms of two or more elements that are chemically bonded to one another.
Elements and compounds are pure substances. A
pure substanceglossary term (opens in a new window) is a type of matter whose samples, wherever found, have the same composition and chemical properties. Mixtures are combinations of pure substances that are not chemically bonded to one another.
Iron is an element and, therefore, a pure substance. An iron sample found in the United States will have the same physical and chemical properties as an iron sample found anywhere else on Earth, or even on Mars.
For example, all samples of iron will be shiny, silvery-gray,
solidglossary term (opens in a new window) at room temperature, have a density of 7.87 g/cm3, melt at 1,538°C, boil at 2,861°C, and react chemically with oxygen to form iron oxide or rust. There are over 100 known elements, and each is a pure substance that possesses its own unique set of physical and chemical properties. An element’s properties are the same no matter where it is found.
The other type of pure substance is a compound. As is true of elements, a sample of a compound possesses a defined set of specific physical and chemical properties. All other samples of the same compound possess the same composition and the same set of physical and chemical properties, no matter where they are found.
For example, hydrogen chloride is a compound made up of one atom of the element hydrogen bonded to one atom of the element chlorine. Wherever it is found, a sample of hydrogen chloride will be a colorless
gasglossary term (opens in a new window) that melts at -114.17°C, boils at
-85°C, has the chemical properties of an acid, and will react with certain metals to form a salt and hydrogen
gasglossary term (opens in a new window).
Teacher Note: Misconception
Students may believe that the common usage of the modifier “pure” when discussing foods (for example, “pure” apple juice) is synonymous with a pure substance. In fact, the term “pure” when applied to food products usually refers to mixtures, which are not, in themselves, pure substances.
