Ask A Scientist© Chemistry Archive

name        Clare M.
status      educator
age         20s

Question -  Can you tell me why bubbles in the bath disappear
when a bar of soap is added? Has it got something
to do with the surface tension of the water?

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Clare,

Your analysis of the problem is correct. Soap weakens water's surface
tension and makes water bubbles less able to support themselves.

Regards,
ProfHoff
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Bubbles have EVERYTHING TO DO WITH SURFACE TENSION. The surface tension of
water is quite large for most non-metallic liquids -- about 72 ergs/cm^2.
It is so large that the surface of a would-be bubble breaks because the
water's surface has no elasticity. Surface active agents [surfactants] lower
the surface tension by preferentially absorbing to the surface of the water.
In general, these agents have a water-loving [hydrophylic] and a
water-hating [hydrophobic] end. So the molecules spread out on the surface
of the water with the hydrophylic "end" in the water and the hydrophobic
"end" sticking out into the air. The forces between these molecules is much
weaker than water itself, making the surface more elastic. Hence, a bubble
forms.

The difficulty you describe is that the elasticity that the surfactant
provides is very molecule-dependent, so that it is possible that the
addition of a poorer surfactant which is preferentially absorbed to the
surface of the water actually displaces the better bubble former. As a
result, the bubbles "break". It becomes a "defoamer".

I do not know what grade level you teach, but if you search the Internet for
"bubbles" or "bubble formation" or similar key phrase you will find an
abundance of information, including bubble organizations, to satisfy your
students' knowledge from K through 12.

Vince Calder
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