I found a glass jar labeled potassium metabisulfite in my abandoned photo lab. The jar has been standing with a loosely fitting lid for thirty years in a cool and dry place.
The jar contains a dry substance now like matte flakes. The substance does not react visibly to hydrochloric acid and it does not smell of sulfur dioxide.
What substance has the potassium metabisulfite turned into? Just curious.
EDIT. The substance does not dissolve visibly in cold or boiling water. A few flakes poured in water remain on the bottom seemingly unaltered.
MORE EDIT. When heated to red heat in a test tube, it does not melt, the coarse powder is not sintered. No water is given off. Some sulfur is sublimated, a yellow deposit in the cold part of the test tube and a smell of sulfur. Not much sulfur is given off, the volume of the original substance is not visibly altered.