[William]

William asked the Naked Scientists:
   
Dear Dr Chris,

I am a teacher and we recently had an outside speaker who got the students to do a science experiment where vinegar was added to baking soda and washing up liquid to make a basic, home-made fire extinguisher to put out a fire.

The bubbles were not very effective in putting out the fire, but they then added liquorice, which made the walls of the bubbles much stronger. The bubbles did not pop for a long time and they put out the fire with no problem.

Any possible explanation for this?

William in Sussex

What do you think?

[RD]

The soap bubbles pop when the soap and water separate out.

 [ Invalid Attachment ]
http://www.creativereview.co.uk/commissions/planet-tozer


Adding a viscose substance like glycerine, or syrup increases the time the water and soap takes to separate out,
 giving longer lasting bubbles. 

[Bored chemist]

It's tricky to see how the soap and water would separate- they mix spontaneously.
I understood that the normal cause of death of bubbles was evaporation. Adding a humectant like sugar or glycerine retards the evaporation of water.
In this case I suspect the licorice is acting as a surfactant (along with the washing up liquid).

[RD]

It's tricky to see how the soap and water would separate

I thought this was quite a good illustration of soap separating out ...

 [ Invalid Attachment ]

The soap can be seen forming clumps, leaving a thinned transparent top to the bubble, (appears black), where it will fail.

[Bored chemist]

I think it's an illustration of the liquid draining down the bubble leaving it thinner at the top.

[RD]

I think it's an illustration of the liquid draining down the bubble leaving it thinner at the top.

Yes, but the film is not homogeneous: clumps of soap are forming: soap is seperating from the water.

[Bored chemist]

I think it's more like this
http://www-math.mit.edu/~bush/tears.html
the water is leaving they system, but unevenly, and the surface tension effects that are produced give rise to the swirls you can see.

If I leave a bar of soap in a sink full of water they mix- why would they decide to separate later?

[RD]

Thanks for that link BC : I’ve now gone cross-eyed trying to read the blue text on a red background. []

[Bored chemist]

If you highlight it it becomes more readable.