What transforms brown spots on a TLC plate into visible signs when placed in an iodine chamber?

The transformation of brown spots on a TLC plate into visible signs within an iodine chamber occurs due to the formation of colored complexes. When the TLC plate is placed in an iodine chamber, iodine vapor interacts with the compounds present on the plate. This interaction leads to the formation of complexes between the iodine and the compounds, resulting in the appearance of colored spots that are often easily visible against the background of the TLC plate.

Iodine has a strong tendency to interact with organic compounds by forming charge-transfer complexes, which can alter the color and visibility of those compounds on the plate. This mechanism is particularly effective for compounds containing functional groups like double bonds or heteroatoms that can interact with iodine, enhancing their visibility for analytical purposes.

While the first choice mentions adsorption, which is a part of the process, it does not entirely capture the formation of these colored complexes that turn the brown spots into visible indicators of the compounds present. Thus, connecting the chemical interaction with the resulting visual change provides a comprehensive understanding of why the iodine chamber is effective in making those spots visible.