Before we leave this 1st section, let me ask you a question. You have certainly encountered in textbooks or newspapers the following two expressions:
Chemical substances and chemical species, do these express the same reality or not? Justify your answer and if necessary illustrate by examples.
Solution:
Chemical substance and Chemical species are closely related concepts but not synonymous.
Chemical species are chemical entities such as an atom, molecule, ion, radical etc.
A Chemical substance is a chemical entity that can be identified and isolated, i.e. can have an independent existence. It has its physical properties such as melting and boiling points, density, etc. and chemical properties. A chemical substance is identified by its chemical formula.
Examples of chemical substances:
water (H2O), sodium chloride (NaCl), ethanol (CH3CH2OH).
Rare gases exist as isolated atoms, they have physical properties and chemical properties (low tendency to react), and therefore rare gases are chemical substances.
In addition to chemical substances, according to the definition of chemical species above, we have other chemical entities which do not present all the properties of a chemical substance. The presence of those chemical entities can be identified by different methods, but they cannot be isolated individually, in other words they do not have a stable independent existence. They have chemical properties and are represented by chemical formulas or symbols.
Examples of chemical species which are not substances:
hydroxide ion (OH-), hydrogen ion (H+), metal cation (Cu2+), halide ion (X-), polyatomic ions (MnO4-, CO32-), atom (Cl, O), radical (H3C.).
The sources of those species can be: dissociation of ionic substances, decomposition of chemical substances, ionization of molecules, etc... They participate in multiple chemical reactions to form new chemical substances or other chemical species.
Examples:
H+(aq) + OH-(aq) → H2O(l)
Ag+(aq) + Cl-(aq) → AgCl(s)
NH3(aq) + H+(aq) → NH4+(aq)
Some cations and anions interact with light to give specific colors to their compounds or aqueous solutions. This is particularly seen in transition metal compounds.
Examples:
Cu2+(aq): blue
MnO4-: violet
CrO42-: yellow
Therefore, the word chemical species includes all chemical entities, including chemical substances.