A substance is defined in REACH by Article 3 and in CLP (The CLP Regulation ensures that the hazards presented by chemicals are clearly communicated to workers and consumers in the European Union through classification and labelling of chemicals) by Article 2 as:
“a chemical element and its compounds in the natural state or obtained by any manufacturing process, including any additive necessary to preserve its stability and any impurity deriving from the process used, but excluding any solvent which may be separated without affecting the stability of the substance or changing its composition”.
The definition thus goes beyond a pure chemical compound composed of a single molecule. The term covers both substances obtained by a manufacturing process and substances in their natural state and which can both include several constituents within the substance and to be taken into account as far as possible when identifying the substance for REACH and CLP purposes.
According to REACH and CLP a substance may contain:
one or more main constituents: constituent(s) that make(s) up a significant part of that substance; the main constituent(s) should clearly be other than the following:
impurities: all the unintentional constituents coming from the manufacturing process or from the starting material(s). These could be the result of secondary or incomplete reactions occurring during the production and are present in the final substance even if not sought by the manufacturer.
additives: all the constituents which are intentionally added to stabilize the substance and only for this purpose. The reader has to carefully consider the difference between a substance and a mixture. A mixture consists of several substances. Each individual component substance in a mixture needs to be identified, registered according to REACH and, when required, notified according to CLP either by the substance manufacturer or by the importer of the mixture.
Source: European Chemicals Agency