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Name: Mylar
Status: Student
Grade: 9-12
Location: TX
Country: N/A
Date: November 2005


Question:
Why is it that gasoline has larger molecules than water molecules, and yet gasoline can escape through holes that are too small to pass water?



Replies:
The reason for this is water has a very high surface tension (72ergs/cm^2) compared to hydrocarbons (~ 21 ergs/cm^2). Remember that the size of the "holes" you are referring to (say 0.001 to 0.1 mm) is much larger than the size of either water or hydrocarbon molecules, so the size of the molecules does not come into play. The ease with which a liquid will pass through a large hole is complicated, but an important factor is how well the liquid "wets" the walls of the hole. This "wetting" in turn is proportional the inverse of the surface tension (approximately). So the LOWER the surface tension the easier it is for the liquid to pass through the hole.

This effect also comes into play in another context. Soaps reduce the surface tension of water significantly. This allows "soapy water" to get into the cracks and crevices of a surface. This plays an important part in the ability of "soapy water" to remove dirt from surfaces.

Vince Calder



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