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what is the surface tension of boiling water

Water molecules consist of one oxygen atom and two hydrogen atoms (H2O). Because the hydrogen atoms are aligned at an angle, the orbiting electron cloud surrounding the molecule is uneven and exhibits polarity (a little like the polarity of a magnet). When water molecules attract each other the polarity affects how they join, especially at the surface of a liquid. It is as though they 'joined hands' and tension (a pull) is necessary to separate them. This provides a water surface with 'surface tension' which can float a needle, etc. as you point out. The polarity of water molecules also draws the water up into a thin tube (capillarity) exhibiting a meniscus. Water (unlike mercury) is attracted to glass. Snow flakes are each unique because of the unlimited ways the polarized water molecules can join (by the billions) when they freeze.

Adding a soap or detergent prevents the water molecules from 'holding hands' and their surface tension is lost. In fact soaps and detergents are wetting agents that allow water molecules to penetrate dirt and help remove it (from clothes, etc.).


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