why we just see a light spot of benzaldehyde after tlc in iodine chamber which soon vanishes and does not appear again?
The phenomenon you described is due to the volatile nature of benzaldehyde and its reaction with iodine in the TLC (thin-layer chromatography) process. When benzaldehyde is placed on a TLC plate and developed in an iodine chamber, it reacts with iodine to form a colored complex. This complex appears as a light spot on the plate.
However, benzaldehyde is highly volatile, meaning it readily evaporates at room temperature. As a result, the light spot you observe quickly disappears because the benzaldehyde evaporates from the TLC plate. Since it is no longer present on the plate, there is no longer any material for iodine to react with, and thus the original spot does not reappear again.
In order to preserve the spot and prevent the rapid evaporation of benzaldehyde, you can try using a different developing reagent or sealing the edges of the TLC plate to limit exposure to air. Additionally, you may consider using other detection methods for benzaldehyde, such as UV-visualization or staining with specific reagents that can provide more stable and longer-lasting results.