Specifying that the standard state is a specific temperature and pressure, would mean that the standard state is not even remotely realizable for many gases. Pure Substances have sharp melting and boiling point and have standard melting or boiling point, contrary to this, boiling and melting points of mixtures varies by the proportion of constituents. Homogenous in chemical aggregation means that the chemical elements must be combined. chemical substance - material produced by or used in a reaction involving changes in atoms or molecules chemical explosive - a chemical substance. A pure substance is in the purest form and has no impurities in it while mixture has impurities or is made up of two or more than substances. Figure 3-0-1: Illustrating the definition of a pure substance. A pure substance does not have to be of a single element or compound. For a pure substance the concept of standard state applies to the substance in a well defined state of aggregation at a well defined but arbitrarily chosen standard pressure. Pure substance synonyms, Pure substance pronunciation, Pure substance translation, English dictionary definition of Pure substance. A substance that has a fixed chemical composition throughout is called a pure substance such as water, air, and nitrogen. For a solute in solution it is the (hypothetical) state of solute at the standard molality m°, standard pressure p° or standard concentration c° and exhibiting infinitely dilute solution behaviour. For a pure phase, or a mixture, or a solvent in the liquid or solid state it is the state of the pure substance in the liquid or solid phase at the standard pressure p = p°. Three standard states are recognized: For a gas phase it is the (hypothetical) state of the pure substance in the gaseous phase at the standard pressure p = p°, assuming ideal behaviour. State of a system chosen as standard for reference by convention.
The IUPAC definition of standard state is: