Water molecules are attracted to each other strongly, which is why they stay as a liquid until heated to a temperature where the bonds between them are broken and they rise into the air as vapour. At the surface of still water, there are no water molecules above pushing or pulling against the surface molecules, so the surface molecules are even more strongly drawn together than usual. This causes them to act as though they form a skin over the surface. It is this effect that is called surface tension.
Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible. Surface tension allows insects (water striders), usually denser than water, to float and slide on a water surface.
At liquid–air interfaces, surface tension results from the greater attraction of liquid molecules to each other (due to cohesion) than to the molecules in the air (due to adhesion). The net effect is an inward force at its surface that causes the liquid to behave as if its surface were covered with a stretched elastic membrane. Thus, the surface comes under tension from the imbalanced forces, which is probably where the term “surface tension” came from. Because of the relatively high attraction of water molecules to each other through a web of hydrogen bonds, water has a higher surface tension (72.8 millinewtons per meter at 20 °C) than most other liquids. Surface tension is an important factor in the phenomenon of capillarity.
Surface tension has the dimension of force per unit length or of energy per unit area. The two are equivalent, but when referring to energy per unit of area, it is common to use the term surface energy, which is a more general term in the sense that it applies also to solids.
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