# eli5: Why does shaking a bottle of soapy water increase the pressure inside?

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If I partially fill a bottle with water (even cold water), add some soap, close it up and shake it; when I open I open it back up, it tends to spurt / hiss / generally act like it was under pressure all of a sudden, even though the total volume of liquid inside is still the same.

Why?

In: 12

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Usually this comes down to water temperature.

If the water is warmer than the air, then shaking it up heats the air causing it to expand. Has nothing to do with the soap, we just usually only put hit water in a bottle, then cap and shake it when we’re also adding soap to clean it.

Add soap to cold or room temp water, let it rest a bit so the temperature really equalizes, then cap and shake. It won’t pop the same.

Good question, changes the vapor pressure?

Imagine a still water bottle. Pour some soap into it… what happens? Nothing really just soap chilling in the water. Now shake the bottle. You have bubbles. Those bubbles are pockets of gas that formed from the interaction of the soap and the water with a little bit of added kinetic energy. The increase in gas will increase the pressure. Now your water bottle has lots of pressure compared to the outside environment. When you pop open the lid the pressure escapes outward causing that spurting/hissing you referred to.

When a bubble forms, it traps a certain volume of air from the ambient pressure. More specifically, a fixed mass of air.

The bubble’s walls do pull slightly on the air trapped in it, raising its pressure. This does change the volume of air, makes that air smaller that the air in the container.

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That should mean the container picks of a small negative pressure, though.