Science Questions

Why does dishwashing soap make bubbles?

Sun, 28th Nov 2010

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Question

Scott Lucero asked:

Hi Chris -

 

I'm a big fan in the States listening to the podcast.† My 10 year old son has a question about soap bubbles.† His question is simple, and I suspect difficult to answer.†

 

Exactly why does dishwashing soap make bubbles?

 

I understand about the soap sort of denaturing the fatty organic matter (software is my strong suit, clearly not chemistry).† I don't understand why the soap would be more prone to make bubbles that water or lemonade.

 

Many thanks,

 

Scott

 

BTW, when I play the podcast for my kids and their friends as I'm driving around, they are always amused and wonder about the whole 'naked' thing. I tell them about the stripping down science, but ultimately resort to the fact that you all are British as an explanation.† Do you have any guidance for what to tell kids about

your show?

Answer

Dave -   It is interrelated with surface tension, but it doesnít work very well with pure water because bubble is a very, very thin layer of water, and with normal water, the surface tension is so strong, it tends to be incredibly unstable.  It will just all form down into a droplet and pop the bubble before itís even really formed.  But what you do to make a soap bubble is around some kind of detergent or soap.  These tend to have a head which really loves water and is very charged and a tail which is very, very oily and doesnít like water at all.  So they're very, very good for dissolving fats when you're doing washing up because all the tails stick in the fat, and so, you'd have something like oil which doesnít mix with water and you surround it with lots of water-loving heads.  So itís then soluble in water and you can dissolve the fats and clean your washing up.  But these also tend to stick on the surface, so all the water-hating tails tend to cover all the surface of water, and deeper inside the water, you tend to get kind of little vesicles, so little spheres with all the tails pointing inwards in the water-loving heads surrounding the outside.  This means that if you increase the surface, just some of these vesicles kind of get pulled up the surface and they can keep creating surface really, really easily.  They stabilise these very, very thin films of water, soap bubbles.  And so, because you've got all this detergent in there, you get this great big thin films of water with a layer of soap on one side, then a little layer of water, then a layer of soap which is really quite stable, and will last a good couple of minutes.

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Scott Lucero asked the Naked Scientists: Hi Chris - I'm a big fan in the States listening to the podcast.† My 10 year old son has a question about soap bubbles.† His question is simple, and I suspect difficult to answer.† Exactly why does dish-washing soap make bubbles? I understand about the soap sort of denaturing the fatty organic matter (software is my strong suit, clearly not chemistry).† I don't understand why the soap would be more prone to make bubbles that water or lemonade. Many thanks, Scott BTW, when I play the podcast for my kids and their friends as I'm driving around, they are always amused and wonder about the whole 'naked' thing. I tell them about the stripping down science, but ultimately resort to the fact that you all are British as an explanation.† Do you have any guidance for what to tell kids about your show? What do you think? scottrenee, Tue, 23rd Nov 2010

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