A Conversation for How Soap Works

pressurized soap bubbles

Post 1


Can anyone explain where the pressure comes from in soap bubbles? For instance, if you take a plastic jar or bottle and fill it half full with warm water and put a small amount of soap in it, then put on a lid and shake vigorously, the contents of the jar will become pressurized. You can tell by squeezing the sides of the jar or opening it, causing the lid to shoot off in a spurt of bubbles. This effect is enough to warp the door off an automatic dishwasher if you're foolish enough to put hand-dishsoap into it instead of the non-foaming dishwasher detergent. Please don't try this to see - stick with the jar experiment.

The jar is full of water, air, soap, and bubbles. The bubbles are made of air and soap. And possibly water, I'm not sure. But the air and soap the bubbles are made of were in the jar before you put the lid in. Since nothing was added to the jar after the lid was added, something must have expanded. The air is the most likely expander, but I can't see how soap makes air expand. Unless the soap is expanding. Either way, I don't know why it expands.

pressurized soap bubbles

Post 2

Baron Grim

I think I know the answer to this one, PuzzleMage.smiley - smiley The key is WARM water. Try the same experiment with cold (below room temp) water and you might see the opposite effect, ie the bottle (plastic) may crumple.
Before you close the lid on the bottle, the air in the bottle is still relatively close to room temperature and the pressure is the same as that of the room air. Now, once you replace the lid to your bottle, you have now locked in a constant volume (negligible, but not nonexistant, volume change due to expansion of the bottle may occur). As you agitate the soap, air and water mixture in your bottle you are heating the previously room temperature air. The soap in the bottle probably accelerates this since it will distribute the warm water through more of the air since it will increase the surface area compared to plain water. Now I was going to cite Boyle's law for this effect, but after a quicky internet search I found the more applicable but related Gay-Lussac's law.(It's been awhile since I've learned this stuff.smiley - winkeye) Which simply states that if volume is held constant (ie by the rigid walls of your container), then as temperature increases so does pressure. Pressure (P) divided by Temperature (T) equals constant (k). smiley - scientistP/T=k.

pressurized soap bubbles

Post 3


No, it's more than that. I did try, and there wasn't a great difference between hot & cold water - and the thing that started me on this question is the warning that one should never use regular dishsoap in an automatic dishwasher - the resulting foam pressure will actually force the door open, resulting in a huge soapy mess all over the kitchen floor. I was told by someone I consider very reliable that he saw this happen once himself, so please don't experiment with it. That's why I phrased it in terms of a soda bottle - I didn't want to be responsible for someone deciding it's an urban myth and trying it.

pressurized soap bubbles

Post 4

Baron Grim

Hmmm <smiley - scientistscratching head>

I know that dish washer fluid and laundry detergent are "low sudsing" surfactants and therefore do not create as many bubbles as normal soap or dish detergent. But I'm trying to figure out any reason that bubbles would "cause" an increase in pressure.smiley - erm Now I'm not going to believe third hand knowledge no matter how reliable you think it may be. (no offense, it's just a general rule of mine never to believe "a friend of this guy I know once... whatever"). I'm trying to wrap my brain around this one. With a certain amount of water, a certain amount of soap, and a certain amount of enclosed space within the washer... why would the pressure build up. Well, technically I suppose the dish washer is not exactly a closed system. You do have new hot water being added to it several times during it's cycles. And with a sudsy foam, any air being heated up and trapped in bubbles would not be able to escape from the washer as easily as "free range" air. I could see an increase of pressure because of this. I'll still need to do an experiment with a soda bottle and room tempurature soapy water to be sure that bubbles by themselves do not increase pressure (I still think it has to be hot water to heat the air in the bottle but bubbles may increase this effect possibly). Erg... where's Mr. Science when you need him.

pressurized soap bubbles

Post 5


He's here.

Being a scientific kind of person, I had to actually go and try this before answering.

Apparatus: one plastic bottle, approx 300ml volume. Some warm water. Some washing up liquid.

Method: add 250ml warm water to bottle. Seal airtight. Shake vigorously. Carefully open bottle, noting any apparent release of pressure.

Now add a couple of squirts of washing up liquid. Seal and shake. Open carefully.

Results: the "control" bottle (no washing up liquid) did not exhibit any pressure release.

The experiment bottle (with washing liquid) DID NOT EXHIBIT ANY PRESSURE RELEASE. On shaking, the void space almost instantly filled with suds, but on opening these suds were clearly at ambient pressure, and did not escape the bottle even slightly.

Conclusion: shaking water in a sealed bottle does not result in increased pressure. (as expected).

Adding surfactant to water and agitating does not result in additional pressure being generated.

Discussion: I assumed that the answer would be that the pressure was generated by the work you do when you shake the bottle. However, it "felt" wrong - as an engineer my gut instinct was that merely shaking the bottle wasn't enough work to raise the pressure noticeably. Also, if this was the case then the soap should make no difference, other than making the excess pressure more noticeable (foaming would be more noticeable than a slight "fffttt" when you open the top).

I'm glad I did the experiment, because it's clear I was right. No additional pressure was generated, whether or not you add the soap liquid.

So we're left with the questions:

1. If someone else has seen additional pressure in a bottle type experiment, where did it come from?

2. What's this stuff about dishwasher doors?

Question 2 can be reasonably simply answered by noting that dishwashers replenish their water, so are far from a closed system.

Question 1 could be answered by wondering what the pH of the water is, what minerals it contains, and if there are any impurities in either the soap or the bottle.

If there was even a little bit of something acidic in the bottle (say, a few drops of coke or some vinegar), and there was even a little bit of the right kind of alkali (say, sodium carbonate) in either the water or the soap, the neutralisation reaction would liberate noticeably quantities of carbon dioxide. This would cause a rapid and noticeable pressure increase - think bottle rocket...

Hope that helps...


pressurized soap bubbles

Post 6

Baron Grim

As I said in a previous post, I did notice a slight pressure release when using hot water. The trick was getting the cap on the bottle quickly before the air in the bottle warmed up. That way the pressure will be coming from the expansion of the gas caused by the temperature increase. My basic thoughts on this came from a Mr. Wizard episode where he performed a classic demonstration which had previously always been misexplained. The basic demonstration uses a boiled egg, a bottle with an opening a bit smaller than the diameter of the egg and a match. Light the match, drop it in the bottle, place the egg on top. What occurs is the match is slowly extenguished then the egg is "sucked" into the bottle. This was usually explained as due to the match "consuming" the oxygen in the bottle. Which of course a load of dingos kidneys. All the oxygen did was run out thus extenguishing the flame sooner. What actually caused the egg to squeeze into the bottle was the drop in pressure inside the bottle caused by the gas in the bottle cooling after the match went out. (Which Mr. Wizard aptly explained.)

By the way Hoovooloo (with the new anonymous monicker) I just stumbled on the discussion of your article's censorship and your remake of your user page (I'm still in the backlog)... but smiley - yikes!!!

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