Surface Tension

SURFACE TENSION 1. A pond skater can walk on water but a person cannot. Why? The surface tension of water acts as stretched membrane due to surface tension. So, it can support small loads. But, this tension is not sufficient to support large loads and the membrane breaks down when such load is kept. Hence, a pond skater which has small load can, therefore, walk on water but a person whose load is very large cannot. 2. A wet tent will let in water if the inside is touched. Why? In wet tent, there is a surface film. The surface acts as a stretched elastic membrane due to surface tension. The membrane allows not more water to enter inside. But when it is touched, the membrane will be broken down and tent will let in water. 3. A small piece of camphor placed on the surface dances. Why? The surface of water acts as a stretched elastic membrane due to surface tension. When a small piece of camphor is placed on the surface, it will lower the surface tension at the point of contact. The camphor particles are pulled towards pure water (i.e. towards the region of higher surface tension). A net unbalanced force arises there and it pulls the camphor across the water. The process continues until the camphor dissolves completely. Due to the pull of unbalanced surface tension, the piece of camphor appears to be dancing on the surface water. 4. Lead shots are made by pouring a molten stream of lead from tower. What is the physical reason behind it? Liquids have a property called surface tension due to which it tries to minimize its surface area. We know for a given volume, spherical shape has its minimum surface area. Liquid, therefore, takes a spherical shape when it is allowed to do so. When molten lead is poured from tall tower, the stream of lead will be broken down into small drops during fall and they will take spherical shape resulting excellent shots. 5. Oil spreads over the surface of water, whereas water does not spread over oil. Why? The force of cohesion of oil molecules is lesser than the force of adhesion between the molecules of oil and water. Therefore, oil spreads over the surface of water. But force of cohesion of water molecules is greater than the force of adhesion between water and oil molecules. Therefore, water does not spread over oil surfaces but takes spherical shape. 6. Small mercury drop is nearly spherical but larger one is flat when resting on clean glass surface. Why? The shape of liquid drop depends upon the surface tension as well as the force of gravity acting on it. The force of gravity depends upon the volume of the drop whereas the surface tension depends upon the surface area of the drop. Therefore, the ration of surface tension to the force of gravity is proportional to . When r is small, the surface tension dominates the force of gravity and the drop is nearly spherical. But when r is large, force of gravity dominates the surface tension and hence the drop becomes flat. 7. The flat glass slides with a thin layer of water between them are difficult to pull apart, why? Water wets the glass, i.e. the force of adhesion between glass and water molecules is large in comparison to the force of cohesion between water molecules. When a thin layer of water is

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introduced between two flat glass plates, the force of adhesion comes into play into large area and they are difficult to separate. A loop of thread in a wire frame containing soap film becomes circular when its inner film is broken. Why? Before breaking the film, the loop was under the action of balanced force due to surface tension. When the inner film is broken, there is no longer the force of surface tension from inside and force becomes unbalanced in outward direction. The film outside the loop will pull the thread into circle, as for a given perimeter, the area of circle is the greatest and hence that of the film outside is the least. What is the effect of impurities on the surface tension of water? When the impurity is added to water is highly soluble, it increases the surface tension because of sharing attractive force of solute molecules. That is why aqueous solution of common salt and zinc sulphate enhances value of surface tension. On the other hand, if the impurities added to water are very less soluble, the intermolecular force decreases and hence the surface tension decreases. Ploughing of the field helps to retain the moisture of the soil. Why? Ploughing of the field breaks the small capillaries formed in the soil. This stops the capillary action of water in the soil. Thus, water contained in the soil cannot evaporate and moisture is retained. Can the waves of sea be calmed by pouring oil on its surface? The oil poured on the sea spreads over the surface which is driven forward by wind. Comparatively pure water is left behind. Since surface tension of pure water is greater than that of oil, it exerts backward pull to water and prevents the wave from becoming very large. Why does hot soup taste better than cold one? Increase in temperature decreases surface tension of liquid. So, the surface tension of hot soup is less than that of cold soup and hence hot soup spreads over larger area, giving better taste. Hence, hot soup tastes better than cold soup