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The Claim

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."

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The Short Version

Chemists commonly use phrases like “the Cu2+ ion is blue in aqueous solution” as shorthand, even though the actual colored species is a hydrated or aqua complex. Reliable references and terminology support both parts of that statement: aqueous ions are hydrated, and the shorthand is widely used. The main caveat is that precise technical writing more often names the hydrated complex explicitly.

Caveats

  • The precise species in water is usually a hydrated ion or aqua complex, not a bare ion.
  • This shorthand is most clearly documented in educational and reference materials; research papers often use more explicit coordination formulas.
  • If precision matters, “hydrated ion,” “aqua complex,” or the specific complex formula is better than saying “ion plus water.”

The Receipts

  1. Water Dynamics in the Hydration Shells of Biomolecules - PMC

    PubMed Central

  2. A Study of the Hydration of the Alkali Metal Ions in Aqueous Solution

    PubMed Central

  3. Iron

    Royal Society of Chemistry

  4. Origin of Color in Complex Ions

    Chemistry LibreTexts

  5. 4.4: Hydration of Ions in Aqueous Solutions

    Chemistry LibreTexts

  6. COORDINATION CHEMISTRY

    University of Texas at Austin

  7. hydrated ion

    IUPAC Gold Book

  8. Hydration of Ions in Aqueous Solution

    MDPI

  9. Problem 113 Most transition metal ions are colored. For example, a solution of CuSO4 is blue.

    Vaia

  10. Transition metals and color

    Khan Academy

+ 9 more sources — see the full list on Lenz

Filed Under

Chemistry