Pure substance

A pure substance, also called a chemical substance or simply a substance, is a homogeneous system of defined and constant composition characterized by specific chemical and physical properties.

A substance consisting of the same atoms (i.e., the same chemical element) is called an elementary or simple substance (e.g., O2, O3, N2, H2), while it is called a compound substance (or chemical compound) if it consists of atoms of different types (e.g., H2O, CO2, H2SO4).

A collection of several pure substances in varying proportions is called a mixture. A substance is never 100% pure and usually contains impurities, sometimes in trace or ultratrace amounts, so that in most cases what appear to be substances are actually mixtures with varying amounts of impurities.

Elemental substances and the forms in which they occur

Elementary substances can exist in different forms:

  • in monoatomic form: the noble gases (He, Ne, Ar, Kr, Xe, Rn, Og);
  • in molecular form: some examples are H2, N2, O2, O3, F2, P4, S8, Cl2, Br2, I2;
  • in the form of a continuous whole (crystalline or amorphous) of covalently bonded atoms: for example, C, Si, B, Sb;
  • in metallic form: most elements are metals, some examples are Fe, Na, Au, Ca, U.

The same substance can exist in different allotropic forms: for example, diamond and graphite (both pure substances formed from carbon) occur in different crystal structures; another example is diatomic oxygen (O2) and ozone (O3), which are both composed of oxygen but differ in molecular structure.

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