Which compound is absorbed by latent print residue but not considered a chemical reaction?

Iodine is absorbed by latent print residue and is utilized in the process of developing fingerprints. When iodine vapor comes in contact with the oils and moisture found in the latent print residue, it adheres to these substances, producing a visible image of the fingerprint. This process is purely physical; the iodine molecules physically attach to the molecules present in the latent print rather than engaging in a chemical reaction that alters either substance’s chemical structure.

In contrast, the other substances listed involve chemical reactions with latent print residue. For instance, silver nitrate reacts chemically with chloride ions in the latent print, creating a visible compound that reveals the print. Ninhydrin interacts with amino acids in the residue, leading to a chemical transformation that produces a color change. Rhodamine 6G also operates through a chemical reaction, fluorescing in the presence of certain components within the latent print. Therefore, when considering the mechanism of action regarding these various compounds, iodine stands out as the only one that indicates a physical absorption rather than a chemical change.