Defined chemical compound with fixed stoichiometry (e.g., H2O, NaCl).
Pure elemental substance (e.g., O2, Fe).
Species defined by a generic or specific chemical formula.
Naturally occurring inorganic solid.
Physical blend of multiple species.
Indicates a self-reaction or internal rearrangement (e.g., tautomerization).
Nature of the chemical entities participating in a reaction.