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Q & A: Why water climbs upward

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Most recent answer: 01/28/2013
Q:
Why does water climb up paper towels?
- CD (age 9)
Damascus, MD, USA
A:

That is a very good question. In order to know why water can climb up a paper towel, we need to know some properties of water. First, water tends to stick to itself. (People call that "cohesion".)   Second, water tends to stick to other things like paper and clothing. (People call that "adhesion".)  The main reason that water sticks to itself and to many other things is that little electrical charges in the water molecules are attracted to charges in other molecules.

The water sticks well enough to the molecules in the paper towel that they pull it right up against gravity, the same way you can pull a piece of paper up stuck to some tape. 

One way to think about this is that a sort of energy is reduced when the water goes up the paper towel. The energy of the uncovered electrical charges in the paper is reduced by getting near opposite charges in the water. Some water molecules get more energy, because they now are at the surface of the towel instead of in the liquid water, but the lowered energy in the paper more than makes up for that. 

Erik (and mw)


(published on 01/28/2013)

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