The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. The molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. This forms a surface 'film' which makes it more difficult to move an object through the surface than to move it when it is completely submersed. The surface tension of water is 72 dynes/cm at 25%B0C . It would take a force of 72 dynes to break a surface film of water 1 cm long. The surface tension of water decreases significantly with temperature. Hot water is a better cleaning agent because the lower surface tension makes it a better 'wetting agent' to get into pores and fissures rather than bridging them with surface tension. Soaps and detergents further lower the surface tension. In the case of the matches, the state of equilibrium is disturbed, a lower force is created between them than that on the other side, this causes them to be pulled apart. http://hyperphysics.phy-astr.gsu.edu/hbase/surten.html
Dec 12 02, 11:53 AM
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