Why does foam form on orange squash?

[wildwildebees]

Why do I get a foam forming on my glass of squash, but not just a glass of water?

i.e.

If I fill a glass with water from the tap, with the water flowing pretty fast, there are only a very few, quite large, bubbles which form on the surface of the water, and they quickly dissipate away.

If I add a small amount of squash (cordial) to the bottom of the glass and then add the water in the same way, a large number of small bubbles (a foam, I suppose) forms on the surface, and takes 3 or 4 seconds to disappear.

Anyone know what's responsible for the difference? It seems related to a soap foam, but I wouldn't have thought there any any ingredients in common...

Cheers!

 

[Drakkith]

Hmm. Could it have something to do with the viscosity of the substance compared to water?

 

[256bits]

And/or
One agent needed to produce foam is a surficant which lowers the surface tension of water. Some ingrediant in the sqaush could midly be providing this feature, So in essence the action is the same as a soap. You could search for more information about foam, suficants, foaming agents.

 

[wildwildebees]

Yes surfactants - presumably these don't actually create bubbles but just allow any air bubbles which enter the liquid to be contained? Since the surface tension is lower, so the restoring force which acts to close the bubble to reduce surface area is lower.

I've had a look around to see what the surfactants could be and couldn't find much - citric acid is present in 3 different liquids I tested, but then I also tried pure lemon juice and that didn't bubble at all! Also, milk does create bubbles. Very confusing. But I suppose this is really getting into chemistry now.

 

[sambristol]

Not surfactants in fact quite the opposite - oils.

the cordial contains oils (in fact that's what gives the flavouring and smell of orange)

The oil is hydrophobic and so stabilise the bubble.

Regards

Sam

 

[256bits]

Not surfactants in fact quite the opposite - oils.

the cordial contains oils (in fact that's what gives the flavouring and smell of orange)

The oil is hydrophobic and so stabilise the bubble.

Regards

Sam

I was under the impression that light oils, having a different surface tension than water, de-stabilise foaming.