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Name: Rachel
Status: student
Grade: 9-12
Location: CA
Country: N/A
Date: 9/5/2005
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.

Replies:
Soap breaks the surface tension of water, there by the water molecules cannot sick together to hold a fog on the mirror.

Grace Fields

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?

Unknown

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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