“Chemists often refer to the color of an ion in aqueous solution (the ion plus its hydration shell) as "the color of the ion" for ease of communication, rather than explicitly saying "the color of the ion plus water."”

This article reviews research on water molecules in hydration shells around biomolecules, showing that the hydration shell is treated as part of the solvated species in aqueous solution. It does not directly discuss color terminology, but it provides authoritative background for why chemists often speak of a species in solution together with its hydration shell.

The results of the LAXS and DDIR measurements show that the sodium, potassium, rubidium and cesium ions all are weakly hydrated with only a single shell of water molecules. The smaller lithium ion is more strongly hydrated, most probably with a second hydration shell present. This is direct primary-literature evidence that aqueous ions are routinely discussed together with their hydration shells in chemistry.

The RSC entry describes aqueous iron(II) and iron(III) species simply as “Fe2+ is pale green in aqueous solution” and “Fe3+ is yellow-brown in aqueous solution” without explicitly distinguishing the hydration shell. It treats the observed solution color as a property of the ion itself, even though in coordination chemistry these are understood to be hexaaqua complexes such as [Fe(H2O)6]2+ and [Fe(H2O)6]3+.

“Complex ions containing transition metals are usually colored, whereas the similar ions from non-transition metals are not… The diagrams show the approximate colors of some typical hexaaqua metal ions, with the formula [M(H2O)6]n+.” “The size of the gap determines what wavelength of light is going to get absorbed… The size of the gap varies with the nature of the transition metal ion, its oxidation state…, and the nature of the ligands.”

In aqueous solution, dissolved ions become hydrated; that is, a shell of water molecules surrounds them. Individual cations and anions that are each surrounded by their own shell of water molecules are called hydrated ions. The notation (aq) indicates that Na+ and Cl− are hydrated ions.

The course notes state: “Aqueous ions are almost always initially coordinated with water, and any other complex actually forms by ligand exchange between water and the new ligand.” They describe these as “hydrated ions” or “aquated complexes,” but elsewhere in teaching contexts such species are often summarized as the ‘metal ion in solution’ when discussing properties like color. This shows that what is commonly called an ion in aqueous solution is in fact a coordination complex with water ligands.

IUPAC defines a hydrated ion as “a solvated ion, the solvent being water.” It emphasizes that species in aqueous solution should strictly be considered as hydrated ions rather than bare ions. While the entry does not discuss color terminology, it implies that the species responsible for observed colors in aqueous solutions are hydrated ions; in less formal contexts, chemists often drop the word “hydrated” and simply speak of the color of the ion in solution.

The book discusses hydrated ions in aqueous solution and notes that in many cases ions are studied with incomplete first hydration shells and sometimes second hydration shells. This is relevant to the claim's underlying concept, but it does not directly verify the specific communication habit about calling the hydrated species simply "the ion."

“Most transition metal ions are colored. For example, a solution of CuSO4 is blue. How would you show that the blue color is due to the hydrated Cu2+ ions and not to the sulfate ions?” “The blue color of the CuSO4 solution is due to the hydrated Cu2+ ions, not the sulfate ions.”

In describing solution colors, the instructor says things like “Copper(II) ions in aqueous solution are blue” and “Nickel(II) ions in aqueous solution are green.” The focus is on the color of the ‘ions’ in water, without explicitly naming the corresponding hexaaqua complexes. This reflects a common teaching simplification where the color of the hydrated complex is discussed simply as the color of the ion in aqueous solution.

“Copper(II) salts in water are usually blue. For example, copper sulfate (CuSO4) in aqueous solution gives a blue colour due to the Cu2+ (aq) ion.” “The characteristic blue colour is associated with the copper(II) ion in solution.”

The colours of many transition-metal ions in solution are due to complex ions formed with water molecules as ligands. The page explains that the colour seen in aqueous solution is a property of the hydrated complex, not a bare ion alone.

“Transition metal complexes often appear colored because the d orbitals of the metal ion can absorb visible light… The color we see corresponds to the complementary color of the light absorbed by the metal ion in the complex.” “For example, aqueous [Cu(H2O)6]2+ appears blue because it absorbs orange‑red light.”

“Students are frequently told that ‘copper(II) ions are blue’ or ‘nickel(II) ions are green’. Such statements are pedagogical simplifications: in aqueous solution the species responsible for the colour are hydrated complexes such as [Cu(H2O)6]2+ and [Ni(H2O)6]2+.” “While this shorthand is common in teaching, it can obscure the role of ligands and coordination environment in determining colour.”

“In aqueous solution, ‘H+’ is not a bare proton but is strongly hydrated. The excess proton is most often described in terms of structures such as H3O+ and larger hydrated clusters.” “In common chemical notation, however, the species is usually referred to simply as H+ (aq), acknowledging implicitly that the proton is associated with a hydration shell.”

One highly upvoted answer states: “What we write as Cu2+(aq) is really a shorthand for some hydrated complex such as [Cu(H2O)6]2+… In practice chemists rarely write the hydration shell explicitly unless it matters; we just say ‘Cu2+ in solution’ and everyone understands this means a hydrated species.” Although an informal Q&A site, this reflects explicit practitioner acknowledgment that the usual ion-in-solution language is a convenience rather than a literal description of a bare ion.

In general chemistry and inorganic chemistry, authors often use shorthand like ‘the color of the ion’ when discussing an ion in aqueous solution, even though the measured species is a hydrated complex or aqua ion. However, in more technical writing, authors usually specify the coordination sphere or aqua complex when the distinction matters.

This educational page defines a hydration shell as a structured arrangement of water molecules around ions in aqueous solution. It is relevant background, but it does not provide evidence that chemists specifically call the color of the hydrated ion simply "the color of the ion."

The speaker explains that when ions dissolve in water, each ion is surrounded by a sphere of water molecules, and that these structures are called hydration shells. The video supports the chemistry background but does not directly address color terminology or the shorthand phrase in the claim.

When we say a solute particle is hydrated it means the same thing as solvated, but we know the solvent is water. These little clusters of water molecules surrounding a solute particle are called hydration shells. This is educational support for the idea that aqueous ions are understood together with their hydration shells.