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?


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

  • 2
    $\begingroup$ There is no gas, it is trapped air. Point to ponder, would a detergent form foam or suds in vacuum? $\endgroup$
    – M. Farooq
    Jun 20 '20 at 12:50
  • 1
    $\begingroup$ 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
    Jun 20 '20 at 16:58
  • $\begingroup$ 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 $\endgroup$
    – The_Vinz
    Jun 20 '20 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
    Jun 30 '20 at 7:51
  • $\begingroup$ 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
    Jun 30 '20 at 9:33

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