Ask A Scientist General Science Archive

name         Rachel
status       student
grade        9-12
location     CA

Question -   I am independently studying condensation, and I
discovered that when you dilute liquid soap by at least fifty percent,
then spread it on the mirror over the bathroom sink that after you take a
hot shower, the mirror does not fog over as it usually does; the soap
somehow prevented that, and I was wondering if someone here could tell me
why that happens.
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Soap breaks the surface tension of water, there by the water molecules
cannot sick together to hold a fog on the mirror.

Grace Fields
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I would like to know more about your experiment. Was the diluted liquid 
soap still wet on the mirror surface when exposed to the vapors from the 
hot shower, or did the soap dry out first? You quoted diluting the liquid 
soap by at least 50% - does the condensation prevention not work if the 
soap is more concentrated? Does the condensation not happen if you just 
wet the mirror surface with plain water?

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On "clean" glass water forms small hemispheres due to its unusually high 
surface tension (about 72 ergs/cm^2 at 25 C. As a result, the water 
droplets scatter incident light and makes the mirror look "fogged". The 
surface of a glass mirror wiped with a soap solution partially dissolves 
in the water condensate, reducing its surface tension to some 25-35 
ergs/cm^2. This causes the water droplets to "spread" into a thin uniform 
continuous film on the mirror and makes it appear as though there is no 
water on the mirror.

Vince Calder
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