How Everything Works
How Everything Works How Everything Works

Question 1002

What is surface tension? — C
The molecules in a gas are independent and only collide with one another briefly before separating again. In contrast, the molecules in a liquid cling to one another so that they always remain in contact. While their mutual attachments aren't as strong as normal chemical bonds, these molecules have reduced their overall potential energies by moving as close as possible to one another. However, the molecules at the surface of a liquid have no neighbors on one side and don't benefit from the full energy-lowering effects of moving as close as possible to other molecules on all sides. Molecules at the surface of the liquid thus have higher potential energies than molecules within the liquid.

Because physical systems tend toward arrangements that minimize their overall potential energies, a liquid tends to minimize its surface area in order to minimize the number of high-energy molecules it has at its surface. This tendency to minimize surface area is the origin of surface tension in a liquid. The liquid behaves as though its surface were a taut elastic membrane. If you poke at a liquid, you can deform its surface but as soon as you stop pushing on it, it will spring back to its original flat or smoothly curved shape. That springiness is the result of surface tension.


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