What term can be used to label the chemical change sodium cation undergoes after dissolving in water?
This is an interesting question. At first glance no chemical change is possible without a new substance formed, given the definition of chemical change consisting in the conversion from "old" to "new" substances.
But, if you consider a chemical system at chemical equilibrium (or far from equilibrium), being that equilibrium is dynamic in nature, then there are continuously occurring chemical conversions.
These chemical conversions don't create "new" substances, because every involved substance is already present in the system, but, nevertheless, a quantitative or net conversion ( chemical reaction ) is possible by changing the external contraints of the system (e.g. temperature, pressure).
Perhaps the term "chemical change", in your question, is more general than "chemical reaction", if we want it to refer to the single species (microscopic) or to substance-macroscopic level, and if we want it to refer to any change (qualitative or quantitative), whereas a chemical reaction is often considered from a phenomenological point of view (macroscopic change with observable evidence of new substance(s) formed).
To illustrate this with an example, let's consider some table salt dissolving in water. We commonly say no "chemical reaction" happens (no new substance formed). But if you take a close view, you will agree that there are new species formed: the hydrated ions. This change can be described at its best as a chemical conversion, a "chemical change".
In this case, the hydrated ions are generally considered new species, but not new substances. According to the IUPAC definition of "chemical substance " they can't be separately isolated to appreciate any property, because their electrical charge, and they haven't a unique and constant composition.
I can put here two more examples.
Ionisation of an acid is everywhere considered a chemical change or a chemical reaction, but you don't create any "new substance" by putting an acid in water. You consider that a water solution of the acid you dissolved.
Ionisation of hydronium species in water.
#H_3O^+(aq) + H_2O(l) ⇌ H_2O(l) + H_3O^+(aq)#
The equilibrium constant of this reaction amounts to 55,4. We see the two species involved in the chemical reaction undergo opposite chemical changes producing the same couple of products. So there is neither a "new" substance (there is only a substance there: water) nor a "new" species formed. Yet, it is a chemical reaction and a double chemical change because a covalent bond is broken and another one is formed.
I wouldn't give it any term, because it isn't a chemical change at all. It would be labeled "no reaction", or "NR".
Chemical changes must result in formation of new substances, those that result from literal bond breaking and bond making, such that chemically-different substances are formed.
For instance, simply forming hydration spheres is not indicative of a chemical reaction; that is merely the motion of substances in a solvent to surround solutes and establish an energetic minimum. Furthermore, the former state does not exist for a practical amount of time.
#"Na"^(+)(aq) + 6"H"_2"O"(l) rightleftharpoons ["Na"("OH"_2)_6]^(+)(aq)#
is not a chemical reaction, because in this example, it is an ion-dipole interaction, an intermolecular force... and that does not make these substances behave in a chemically-different manner. But chemically different from what...?
All we have done is be more specific about the ion-solvent environment. This always, always happens when you put
We could call it solvation I suppose... but simply because hexaaquasodium is formed doesn't mean it is chemically different from