 The force with which surface molecules are held is called the surface tension of the liquid  It is the force acting perpendicularly inward on the surface layer of a liquid to pull its surface molecules towards the interior of the fluid  It keeps the surface like a stretched membrane, and hence keeps the contact area minimum

 Water striders use surface tension to walk on the surface of pond. The surface of the water behaves like an elastic film: the insect's feet cause indentations in the water's surface, increasing its surface area and thus decreasing surface tension at those points. Its tiny mass and geometry of its legs allow it to be supported by the high surface tension of water  Formation of drops occurs when a mass of liquid is stretched. Water adhering to the tap gains mass until it is stretched to a point where the surface tension can no longer bind it to the tap. It then separates and surface tension forms the drop into a sphere. If a stream of water were running from the tap, the stream would break up into drops during its fall. Gravity stretches the stream, then surface tension pinches it into spheres

 Surface tension at liquid-air interface: A soap bubble is a thin film of soapy water enclosing air that forms a hollow sphere. Surface tension causes a bubble to assume the smallest surface area to contain a given volume -- resulting in the spherical shape  Liquid-solid interface Beading of rain water on the surface of a waxy surface, such as a leaf. Water adheres weakly to wax and strongly to itself, so water clusters into drops. Surface tension gives them their near- spherical shape, because a sphere has the smallest possible surface area to volume ratio  Liquid-Liquid interface Separation of oil and water (in this case, water and liquid wax) is caused by a tension in the surface between dissimilar liquids.

 Temperature Surface tension falls with rise in temperature hence increasing the surface area Major reason for using hot water for washing is that its surface tension is lower and it is a better wetting agent  Solute Solutes can have different effects on surface tension depending on their structure: Little or no effect, for example sugar Increase surface tension, inorganic salts Decrease surface tension progressively, alcohols Decrease surface tension and, once a minimum is reached, no more effect: surfactants

 According to this principle substances which lower the surface tension becomes concentrated in the surface layer whereas substances which increase surface tension are distributed in the interior of the liquid  Lipids and proteins effective in lowering surface tension are found concentrated in the cell wall  Soaps and bile salts reduce the surface tension of water while sodium chloride and most inorganic salts increase the surface tension

 Surface tension of plasma is slightly less than water  Emulsifying action of Bile salts Bile salts are surface active. They facilitate action of pancreatic lipase and hence absorption of free fatty acids. Bile salts lowers surface tension of fat droplets  Hay’s test This test is based on the surface tension and is employed for detecting the presence of bile salts in urine (an indication of jaundice). If urine contains bile salts, the fine sulfur powder sprinkled on its surface settles down due to lowering of surface tension. Fine sulfur continues to float on the surface if urine does not contain bile salts

 Laplace’s law states that the pressure inside an inflated elastic container with a curved surface, e.g., a bubble is inversely proportional to the radius as long as the surface tension is presumed to change little.  A common illustration of this phenomenon is that the effort required to blow up a balloon is greatest when the diameter of the balloon is least

 Alveoli can be compared to gas in water, as the alveoli are wet and surround a central air space. The surface tension acts at the air- water interface and tends to make the bubble smaller (by decreasing the surface area of the interface).  Surfactant is a lipoprotein mixture secreted by special surfactants secreting cells i. e. type II granular pneumocytes present in alveolar epithelium

 On the basis of Laplace law, air would be displaced from the smaller alveolus into the larger one and the smaller alveolus would thus become still smaller  This process would continue until the smaller alveolus would collapse entirely while displacing all of its air into the larger one  This process would lead to instability of alveoli

 Instability is prevented by surfactant. As an alveolus becomes smaller the surfactant becomes more concentrated at the surface of alveolar lining fluid. Hence surface tension becomes progressively more less  On the other hand as an alveolus becomes bigger, the surfactant is spread more thinly on the fluid surface. This increases the surface tension  This property of surfactant stabilizes the sizes of the alveoli, causing the larger alveoli to contract more and smaller ones to contract less.  The absence of surfactant in the alveolar membrane of some premature infants causes the respiratory stress syndrome in them  Factors reducing production of surfactant or increasing its rate of destruction may contribute to adult respiratory distresss syndrome