In chemistry, a chemical substance is state of matter characterized by the entities, i.e. atoms, molecules, chemical species, formula units, etc., that it is composed of; characterized by physical properties, such as refractive index, electric conductivity, melting point, etc. [1] A typical chemical substance, such as water, H20, is often defined as a material with a definite chemical composition. [2]

1. (a) Chemical substance – IUPAC Goldbook.
(b) Pure substance – DiracDelta Science & Engineering Encyclopedia.
2. Hill, J. W.; Petrucci, R. H.; McCreary, T. W.; Perry, S. S. (2005). General Chemistry, 4th ed., p5, Pearson Prentice Hall, Upper Saddle River, New Jersey.

Further reading
‚óŹ Lewis, Gilbert N. and Randall, Merle. (1923). Thermodynamics and the Free Energy of Chemical Substances. McGraw-Hill.

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