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I've noticed that when I put dish soap, water and a little air into a sealed plastic bag, and then shake the mixture, it forms soap suds and the bag expands significantly in volume. Why does it expand?

Update:

After being prompted by the comments, I tried to re-create and measure this phenomenon under more controlled conditions, but the bag does not expand.

The less-controlled conditions, where the bag expanded in the past, were:

  • A plastic bag, slightly dirty from crumbs of cooked ground corn, and canola oil (the bag held arepas)
  • A little squirt of Ultra Ajax Triple Action Orange dish soap (not automatic dishwasher soap). The ingredients (and purpose) are:
    Water (Consistency)
    Ammonium C12-15 Pareth Sulfate (Cleaning and Foaming Agent)
    Lauramidopropylamine Oxide (Cleaning and Foaming Agent)
    SD Alcohol 3-A (Controls Thickness and Clarity)
    Sodium Chloride (Controls Thickness)
    Poloxamer 124 (Controls Thickness)
    Fragrance (Pleasant Scent)
    Methylisothiazolinone Benziosothiazolinone (Preservation)
    Pentasodium Pentetate (Maintains Product Stability)
    Sodium Bisulfite (Maintains Product Stability)
    Dyes (Color)
  • Chlorinated tap water
  • A little air
  • Everything at around 30 C (a little warmer than room temperature)
  • Seal the bag and shake. Bag volume increased approximately 2x, with no noticeable change in temperature.

I could observe a similar phenomenon in the past by sealing my palm over the open end of a dirty drinking glass containing dish soap water and air, shaking it, and feeling outward pressure on my palm. I don't recall what the glass was dirty from.

Today, under more controlled conditions, I used a clean bag, and a clean drinking glass, and I could not create any expansion. So I think either the "dirt" in the vessel contributed to the phenomenon, or the tap water was more chlorinated on the days where I observed the phenomenon, and that somehow contributed to expansion. In today's experiments, the water doesn't smell chlorinated.

If I can re-create the expansion later, I will note the conditions as well as I can.

Update 2:

At the request of a commenter, "under more controlled conditions" as described above (and notably where the water does not smell chlorinated), I added table salt, and it did not cause expansion. When the local water authority chlorinates the water again, I will retry the experiments.

Update 3

A similar question has been posted at https://physics.stackexchange.com/questions/617113/why-does-pressure-in-a-thermos-increase-after-shaking-up-hot-water-and-soap

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    $\begingroup$ There is no gas, it is trapped air. Point to ponder, would a detergent form foam or suds in vacuum? $\endgroup$
    – ACR
    Commented Jun 20, 2020 at 12:50
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    $\begingroup$ Never noticed/thought about. But if the V increases the pressure inside does ultimately originates by mechanical forces linked to surface tension. A bubble must form and so does lifting the bag wall. It principle the air pressure inside has to decrease. $\endgroup$
    – Alchimista
    Commented Jun 20, 2020 at 16:58
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    $\begingroup$ Maybe shaking the solution helps releasing gas that was originally dissolved in water. You could trying either shaking water alone before the experiment, or boiling/cooling it before $\endgroup$
    – user32223
    Commented Jun 20, 2020 at 19:13
  • $\begingroup$ What typ of dish soap is it? For manual dish washing or automated? Does the label reveal anything about its content? $\endgroup$
    – FrankS
    Commented Jun 30, 2020 at 7:51
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    $\begingroup$ May you add some numbers about «expands significantly in volume»? And, what about the temperature of the bag / the bag's content prior / during / past the experiment recording these numbers? $\endgroup$
    – Buttonwood
    Commented Jun 30, 2020 at 9:33

1 Answer 1

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There were three probable contributors to the observed gas:

  1. Air expanded after being sealed into the bag with warmer water.
  2. Air was dry outside, then added water vapor came to an equilibrium partial pressure inside the bag.
  3. Dissolved gases were released from the tap water.
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