\(\mathrm{Fe}^{3+}\) can be detected by the reaction: (1) \(\mathrm{Fe}^{3+}+\left[\mathrm{Fe}^{\mathrm{Il}}(\mathrm{CN})_{6}\right]^{4-} \longrightarrow \mathrm{Fe}^{\mathrm{m}}\left[\mathrm{Fe}^{\mathrm{Il}}(\mathrm{CN})_{6}\right]\) Deep blue (2) \(\mathrm{Fe}^{3+}+\left[\mathrm{Fe}^{\mathrm{III}}(\mathrm{CN})_{6}\right]^{3 .} \longrightarrow \mathrm{Fe}^{\mathrm{III}}\left[\mathrm{Fe}^{\mathrm{III}}(\mathrm{CN})_{6}\right]\) Brown (3) \(\mathrm{Fe}^{3+}+\mathrm{SCN} \stackrel{\mathrm{H}_{2} \mathrm{O}}{\longrightarrow}\left[\mathrm{Fe}(\mathrm{SCN})\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\right]^{2+}\) Blood red (a) 1 and 2 only (b) 2 and 3 only (c) 1,2 and 3 (d) 3 only

Detecting the presence of specific ions in a solution is a cornerstone of analytical chemistry and can be achieved through tailored chemical reactions. Each ion can participate in particular reactions that result in observable changes, indicating its presence. In the context of the exercise, the focus is on the detection of ferric ions, denoted as \( \mathrm{Fe}^{3+} \).

When \( \mathrm{Fe}^{3+} \) ions are in solution, performing a chemical test that results in a colored product can confirm their presence. For instance, when \( \mathrm{Fe}^{3+} \) reacts with potassium ferrocyanide \( \left[\mathrm{Fe}^{\mathrm{II}}(\mathrm{CN})_{6}\right]^{4-} \), a deep blue complex forms—a classic Prussian blue reaction used historically in paintings and dyes. Such reactions work based on the affinity of the \( \mathrm{Fe}^{3+} \) ions for ligands, in this case, the cyanide groups arranged in the ferrocyanide ion.

Confirmation of the ion's presence through such reactions reinforces understanding of concepts such as stoichiometry, coordination chemistry, and the unique properties of transition metals.

Complex formation plays a vital role in the identification of ions. When an ion forms a complex, it binds with one or more ligands, resulting in a new substance that can have distinctive physical and chemical properties, such as color.

For example, the reaction mentioned in the exercise involving \( \mathrm{Fe}^{3+} \) ions and thiocyanate under aqueous conditions leads to the formation of a blood-red complex \( \left[\mathrm{Fe}(\mathrm{SCN})(\mathrm{H}_2\mathrm{O})_5\right]^{2+} \), providing a striking visual cue for the presence of \( \mathrm{Fe}^{3+} \) ions. This reaction rests on the ability of the iron ion to coordinate with the SCN ligand forming a charged complex.

The formation of these complexes often involves d-orbital interactions, which can lead to a rich variety of colors depending on the specific ligands and metal ions involved. As students perform these tests, they witness first-hand the theories of coordination complexes and their applications in detection processes.

Color changes are among the most visual and compelling indications that a chemical reaction has taken place. This is particularly useful in the field of qualitative analysis, where the objective is to detect the presence of specific ions or substances.

In our given exercise, the color change is a significant clue. The deep blue and blood-red colors observed in reactions (1) and (3) are vivid examples of how color change can signal the formation of a new substance—in this case, a color-bearing complex that confirms the presence of \( \mathrm{Fe}^{3+} \) ions. The blue and red colors are produced because the newly formed complex absorbs certain wavelengths of light and reflects others, which is the color we perceive.

Students can learn a great deal from these observations—the electronic arrangements in complexes, the interaction of light with matter, and the practical applications of these phenomena in real-world analytical techniques. By linking a color change to an ionic reaction, students make a powerful connection between abstract chemical principles and tangible outcomes.