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I often get my jam out of the refrigerator, and it looks like this:

enter image description here

That is, all the jam has gone to the top, looking as if the jar has been sitting upside down. However, this happens fairly regularly in both my home and work fridge, so I don't think someone is systematically shifting my jam upside-down and then flipping it back before I retrieve it. :P It doesn't happen every time though. The jar tends to be half to three-quarters full when this happens, but I haven't noticed any further pattern (and I always get standard Smuckers strawberry jam).

I found a very scattered discussion with a variety of hypotheses here. Some of the posters suggest some weird bacteria, but this has happened regularly with different jars of jam, so I'm betting it's not that. My hypothesis is that it's some sort of physics-y effect (something about temperature change with the fridge and pressures?). That said, depending on the answer, Physics StackExchange might turn out to be the wrong place to post this...

Any hypotheses? I'm happy to vary my jam habits to test ideas. :D

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    $\begingroup$ This is hilarious and interesting. +1 $\endgroup$
    – Danu
    Nov 3, 2014 at 7:02
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    $\begingroup$ Epic! Where can I buy this Gravity defying Jam- Does it taste any different to the home brand physics abiding one? $\endgroup$
    – Harry David
    Nov 3, 2014 at 9:27
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    $\begingroup$ I know: it's because you live in Australia <rim shot> . I've never seen anything like that (and over the years I've gone thru rather a lot of jam, preserves, peanut butter, and other gooey foods in jars :-) ). $\endgroup$
    – Carl Witthoft
    Nov 3, 2014 at 12:33
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    $\begingroup$ Relevant SMBC comic: smbc-comics.com/?id=3527 $\endgroup$
    – gigacyan
    Nov 3, 2014 at 13:38
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    $\begingroup$ Another curiosity/clarification question: is the little "safety seal" button depressed when this happens? $\endgroup$
    – jkeuhlen
    Nov 4, 2014 at 0:53

1 Answer 1

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I could be wrong, but my guess would be it is caused by a change in temperature.

At a very simple approximation, the air in the top portion of the jar can be modeled using the ideal gas law:

$$PV=NkT$$ or $PV=nRT$ if you're a chemist.

By simple inspection, you can see that if the amount of jam stays constant, and the glass doesn't budge, a drop in the temperature (caused by placing the jar back into the fridge) will lead to either a decrease in the pressure of the air in the top part of the jar or a decrease in it's volume. Let's assume that the lid isn't air tight and the pressure in the system is constant. This means the volume of the gas inside will be decreasing. Because cold air is more dense than hot air, the gas will decrease in volume from the bottom (near the jam) up.

This decrease in volume of the gas creates a sort of vacuum that pulls the jam up to fill the void of the gas. Because the lid is not completely air tight, some of the gas will escape until eventually the last of the air leaves (or some sort of equilibrium is reached) and the jam sits in the top part of the glass.

So why does it stay there? My guess is this is a characteristic of the jam itself now. Someone else may be able to expand on this further, but the jam will stay near the top now for the same reason that it will stick to the sides without falling down: high surface tension.

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  • $\begingroup$ Cold air is more dense than hot air. Also, if your lid isn't tightly shut, $P$ and $V$ stay constant - $N$ changes with $T$. $\endgroup$
    – pho
    Nov 4, 2014 at 1:07
  • $\begingroup$ Fixed the density typo. In regards to the ideal gas law, I'm making a couple different approximations. Really, I should say that the lid is nearly airtight so that the volume of the gas decreases THEN as the jam pushes up some of that air leaks out. $\endgroup$
    – jkeuhlen
    Nov 4, 2014 at 1:13
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    $\begingroup$ Once the jam is pressed against the lid, the naturally thick jam could possibly act as a sealant, further air-proofing the jar? $\endgroup$
    – CoilKid
    Nov 4, 2014 at 3:20
  • $\begingroup$ It's very possible (maybe likely) that I don't seal the lid very tightly. $\endgroup$
    – Cannoliopsida
    Nov 4, 2014 at 5:05
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    $\begingroup$ This explanation does not make sense. Air doesn't pull; it can only push. The only way that decreasing air pressure above the jam can cause the jam to elevate is if there is air below the jam pushing it up. Even pumping all of the air out of the jar would leave the jam sitting at the bottom with a vacuum above it. Plus, as the air leaks out, it can only do so until the pressure inside the jar is the same as atmospheric pressure. A vacuum or any other large pressure differential cannot be created in a container with a leak. As the air cools and contracts, it would pull outside air in. $\endgroup$
    – Mark H
    Dec 8, 2020 at 4:17

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