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Q & A: What is Surface tension ?

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Q:
What is Surface tension ?
- ritush (age 7)
Edmonton, Alberta, Canada
A:
Ritush,

A liquid, like everything else, is made up of tiny particles, usually ones called molecules. The molecules in a liquid are attracted to each other, pulling on each other, which is the reason water stays together when you pour it into a glass. (If they weren't attracted to each other, they'd fly apart and not form a liquid.)

Think of it this way: If you were a water molecule in the middle of a glass of water, you would be surrounded by other water molecules on all sides, top and bottom. You would hang on to all of these other neighbors. Now, if instead you were a water molecule on the surface of a glass of water you would have other molecules beside and below you, but not above. Making the surface bigger makes more molecules like that, with less neighbors than they could have if they weren't at the surface. So making more surface requires pulling molecules apart. It takes more energy to make more surface, so the molecules pull together to reduce the surface area. That pull is called the surface tension. A free-floating drop forms a sphere because that's the way the molecules can pull together to make the least surface, for a given total volume.

Mats, changed by Mike W.

(republished on 08/01/06)

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