I stored resorcinol in a transparent plastic container with an airtight lid. There was some air captured inside the container too. After some days, it turned brown. please tell what happened to it?
Essentially, this is a almost a duplicate of a similar question on phenols. Resorcinol is sensitive to light, if impure, and also, that reference states it's readily oxidized in the presence of water, especially in alkaline conditions.
See Nilay Gosh's fine answer for a more detailed explanation.