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Surface tension is a property found in many liquids at the air-liquid interface. It is what causes polar liquid solvents like water to form beads on a non-porous surface ran than a thin uniform layer. Surface tension is caused by intermolecular forces within the bulk of the liquid solvent. In the case of water, the main intermolecular binding force is hydrogen bonding. The partially positive hydrogen atoms are attracted to neighboring partially negative oxygen atoms. This highly attractive force is what gives water such a high boiling point for such a low molecular weight molecule. While the hydrogen bonding force is evenly distributed in the bulk of the solvent, at the surface (the liquid-air interface), the hydrogen bonding forces are only present in the interior. This results in the surface molecules being drawn inward and the liquid at the surface beading up into droplets. In a solvent like ethanol, however, hydrogen bonding forces are much weaker so the solvent doesn't bead up into droplets nearly as much as water. The effect of surface tension in water is what allows very light insects like mosquitoes to rest on the surface of the water without their legs or bodies breaking through the surface and getting wet.
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