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Occasionally when I make coffee in my french press I experience something odd. It happens pretty infrequently but certainly enough to be curious about. I have the grounds ready in the carafe. The water just heated in an electric kettle. I pour the water over the grounds and place the plunger on top. Then, maybe 1/4 inch into plunging, the water "explodes" out the top like a busted sprinkler head.

I'm curious what's going on here. Is the slight pressure increase (it can't be much) - or simply the plunging action - adding enough energy to the system to induce a flash boil? I've ruled out that the additional volume that the plunger itself adds is not enough to make the coffee overflow (this is a normal setup in every other respect where plunging would simply deliver a refreshing hot beverage).

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I've experienced something similar. I'm not 100% sure if it's the same phenomenon you're describing, but I suspect so. It happens if you use coffee that's ground too finely, and doesn't have to do with boiling. What happens is that the fine coffee grains block all the holes in the mesh. This means that the water is under more pressure than usual, since it can no longer pass through the plunger. Because of this the water ends up escaping by forcing a small part of the mesh away from the side of the carafe and squirting out at high velocity. The reason for the high speed is just that it's passing through a small aperture - it's the same effect as when you put your finger over a hose.

To prevent this from happening, you could try a coarser grind, or if you already use coarse-ground coffee, try pressing even more gently on the plunger. If you meet resistance then try lifting the plunger slightly before continuing - this should redistribute the grounds slightly and hopefully unblock the holes in the mesh.

From a physics point of view it's worth mentioning that boiling would be a very unlikely response to compressing hot coffee. It is possible for a liquid to be in a "superheated" state, where it's above its boiling point yet remains liquid. When water is in such a state it can indeed boil very suddenly. But this state can only be reached if there are no nucleation sites available to allow steam bubbles to form, and the coffee grounds would probably provide excellent nucleation sites, so if the water were superheated it would boil as soon as you poured it onto the grounds. (This would probably produce very bitter coffee.) Liquids can also suddenly boil if their boiling point decreases below their temperature - but pressing the plunger increases the pressure, which increases rather than decreases the boiling point. This is true for all liquids (by Le Chatelier's principle), so we would never expect boiling to result from an increase in pressure.

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Not sure I should have the authority to mark this correct, but this is probably what's going on. Thank you for the in-depth and very interesting answer! –  xanadont May 9 '12 at 13:37
    
+1 An excellent answer: experimental physics strikes again :-) –  John Rennie May 9 '12 at 14:45
    
D'oh and I was just preparing an answer based on an instanton contribution to a phase transition :-) –  twistor59 May 9 '12 at 14:54
    
BTW--Microwaves and glass vessels can result in superheating and represent a risk in the kitchen. My one personal experience with this behavior sent scalding cream to the ceiling and about eight feet in every direction. My better half and I both received numerous minor burns. Unpleasant but not disastrous. –  dmckee May 9 '12 at 19:07

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