Ask A Scientist Chemistry Archive

Question:
Why do jars of dessicants seal so tightly when allowed to
stand for a long time?
 stan r natonek

Answer:
   I don't know, but my first guess would be as follows: one would expect
there to be normally a few-micron-thick layer of microscopic debris on the
surfaces of the cap and jar that meet.  After the cap is put on, this layer
will be dried out by the dessicant, starting from the point where cap and
jar meet that is closest to the inside of the jar.  As the layer becomes
progressively drier it becomes less easy to deform, and forms more of a
solid plug than a soft lubricating film between cap and jar.  The force
required to put on the cap, when the film is soft, would be less than the
force required to remove the cap, when the film is hardened.  You could
test this guess by seeing if the increase in force required to remove the
cap after a fixed period of time is smaller when you thoroughly clean the
cap and jar threads with reagent-grade acetone (to remove dirt and water)
before putting the cap on.
 christopher grayce