Why Does Shaking Water + Soap Increase The Pressure In The A Closed Container?

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This has never made sense to me. If I put a drop or two of washing-up liquid (“Dish Detergent” in US English) in a plastic food container with water, and snap on the lid, then shake the container, the lid will try to pop off on its own. Or if I do it in a screw-top container like a bottle or a jar, when I go to unscrew the lid it will act like a carbonated beverage (pop!). Why?

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If you’re washing dishes I would assume you’re using warm water which will heat up the air in the container you’re washing and cause it to expand slightly

Bubbles take up more volume (space) than liquid and gas do separately. If you try to increase volume in a fixed-volume container, it instead raises the pressure as it tries to expand.

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I assume that you are filling a room temperature container with warm soapy water, right?

Two things happen here. First, air expands slightly when warmed up. Second, some water evaporates (even if the air was already at 100% humidity which is rare unless you’re doing it in a shower, warming up air increases its vapour-holding capacity) which further increases the pressure.

Soap does not play any role in this, but it makes the pop more noticeable because of the bubbling around the lid. I’ve just tested it with the screw-lid bottle and the effect is similar whether I add detergent or not.

Here’s my $0.02 but honestly I don’t know. Tap water has dissolved air in it. To see for yourself fill a clear glass with water and leave it overnight then you’ll see some bubbles. Shaking it with washing-up liquid releases the dissolved air.