Take a Tupperware container, fill it halfway with water, and add some dishsoap. Put the lid on and shake. Soon pressure will build up and pop the lid off. What I want to know is, how exactly is the mechanical action of shaking translated into a pressure buildup?
I think we need some more data. Does the pressure change occur without the dishsoap? What temperature is the water you’re using? What about using a Rubbermaid container? If I wasn’t at work, I’d probably go play with this right now 
I’ve seen this happen when rinsing out empty milk jugs. If I put warm water into it (no soap), screw on the top and shake it, there’s a definite increase in pressure caused by the temperature increase of the air (shaking just speeds up the heat transfer). Under constant volume, the pressure is directly proportional to the temperature.
well, at least for ideal gases, and the temperature is measured in Kelvin, but if we let details get in the way, we won’t make any progress…
Also, agitating water might force dissolved gases out of solution, but I’m less sure about that. Maybe the soap binding to the water molecules displaces some air that was in solution?
Shaking creates bubbles by mixing air into the water-soap mixture. Since the soap bubbles are much more persistent than bubbles in plain water, they accumulate rapidly with sustained shaking. With each buble formed, a small amount of the soap solution is bound up, essentially increasing its volume, and the pressure of all the bubbles together eventually is enough to pop the lid.
Q.E.D.: WHAT???
I don’t think your science is sound.
And your better explanation is…?
OK, I think that we have to think of this as more of a chemical reaction, then physical.
When you mix soap with water, part of the soap molecule is attracted to the water, while the other part is repelled. That is how soap works to get things clean. I’m guessing that during this process gas is released. Now when the chemicals are agitated, this causes chemicals to react faster and more gas to release. This is why when in an enclosed space pressure builds up.
So shaking the container full of soap and water causes the soap and water reaction to speed up, filling it with gas which makes the top pop off.
Just a guess, but it makes sense to me…
Um, no. There is no chemical reaction when soap dissolves, and no gas is released.
Q.E.D., your explanation leaves me unsatisfied. Perhaps I’m missing some implication, but, as I see it, the bubbles are filled with air that was already in the container, and the bubbles themselves are made of soapy water that was already in the container. Where is the increase in volume?
I think the pressure is mechanical in nature; the bubbles themselves, filled with air and buoying themselves out of the water, push the previously formed bubbles to the top of the foam. These older bubbles deform themselves against the walls and lid of the container; the deformation force leads to pressure.
Poor choice of words on my part. I was trying to conceptualize…well, you ever put too much soap powder in the washing machine?
Try the experiment again, but use cold water and post the results.
Here’s my explanation(s). One of two things is happening:
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The air in the container is simply being heated by the agitation and is expanding.
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Not sure about the science here, but I pose it as a possibility… Water vapor is being created, adding to the vapor content of the system. This is essentially room temperature steam; and like steam, water vapor is more voluminous than the water it came from.
Hmm… sciguy and stuyguy in the same thread. Freaky.
I think stuyguy may have it with explanation 2. But the experiment needs to be rerun with room temperature water, not cold or warm water.
Soapy water lubricates better than plain water, so it may also be lessening the force needed to pop the top.
Except in that case, there is no seal to prevent additional air from being drawn in to create new bubbles. In the sealed container, it can only make bubbles with the air already in the container (and any water vapor created).
I was trying to suggest that it is not the soap that causes the effect, but the water temperature. I’m thinking that the water you are using is warmer than the air and shaking heats up the air causing it to expand.
Maybe its something to do with the surface tension of the bubbles?
Bubbles don’t pop because the surface tension keeps it together. So maybe as you create more and more bubbles the amount of volume taken up by the bubbles increases until it fills the container. Once the container is full of bubbles smaller and smaller bubbles are created, which would all push outward on the container walls.
Just a guess…
If you take a given volume of air and a given volume of soapy water, you would expect the combined volume to decrease slightly when you made bubbles because the surface tension of the bubble compresses the air inside slightly. That is, the pressure inside the bubble would go up because of the surface tension but the pressure of the system as a whole would decrease because the compression of the air in the bubbles decreased the volume. However, for soap bubbles, the compression provided by surface tension is very small and this would not be noticable in a typical container. The bubbles definitely push outward on the container, but no more so than the “unbubbled” air they displace.
I have to agree with DrMatrix that it was likely a temperature effect.
You don’t need soap to do this – as everyone learns in introductory chemistry lab, agitating virtually any interacting liquids or liquid and solid creeates pressure. When you agitate any such mixture in a closed flask you are supposed to vent it (away from your face) every so often to release the pressure. This is true even for mixture of immischible liquids, or for dissolving powders in solution. Exactly why the gas pressure builds up I do not know, but it certainly does. Heck, I see it happen when I’m rinsing out my milk containers for recycling. (Try it. If you’ve got a milk container of the sort with a hermetically sealable closing on it add water, close tightly, and shake. When you open that lid again you’ll hear an obvious and clear release of pressure. Heck, you won’t have to do even that – you’ll ifeel* the increase in pressure from the way the sides bulge out.)
Why does it happen? Damned if I know. Some manifestation of le Chatelier’s principle, maybe. Thermodynamic requirements – mixing stuff up liberates thermal energy because of the increase in disorder that then liberates gas, maybe. But I see it happen all the time.
Well i did the experiment again with precision instruments including:
my disposable resealable plastic container, which previously contained tuna casserole
an organic temperature sensory device (my hand)
the sink in the break room at work
an organic volumetric sensory device (my eyes)
dishsoap
Results:
- Cool water, no soap: some increase in pressure (container bulged outward a little)
- Cool water, soap: approximately the same increase in pressure
- Hot water, no soap: a larger increase in pressure, but no lid popping off. This is probably because the disposable container is more flexible than the permanent ones at home.
- Hot water, soap: approximately the same increase as 3.
Conclusions:
It seems to be the agitation that causes the pressure increase, as suggested by CalMeacham, and not so much the presence or absence of soap. Case 1 especially suggests that agitation introduces thermal energy, resulting in higher air pressure.
I did some experiments last night also, using instrumentation of equivalent accuracy.
Cold water, no soap: decrease in pressure.
Room temperature water, no soap: slight increase in pressure
Room temperature water, with soap: slight increase in pressure (no appreciable difference due to the soap).
Hot water, no soap: Large increase in pressure.
Error analysis: I couldn’t really tell if the “Room Temperature water” was room temperature or just close. Since water tends to feel cooler than air, water may have been a little warmer than the air.
My conclusions: Temperature is the dominant factor. Soap is irrelelevant. Agitation may have a small effect, but the data are inconclusive.