Does Surface Tension Of Water Increase Or Decrease When Salt Is Added To It?


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Iris Phillips answered
Surface tension, the property allowing the surface of water to resist external forces, is the result of the cohesion of similar, or like, molecules. Some of the effects caused by the surface tension of ordinary water can be seen every day. Rainwater beading on waxed surfaces, such as cars, for instance, is one of these effects. This beading is due to the fact that water does not adhere well to wax, but does adhere strongly to itself. As a result of the surface tension, clusters of drops are created. These drops are almost completely spherical, because spheres have the smallest possible ratio of surface area to volume.

Another good example of surface tension in action is a pond-skater's ability to 'walk on water'. The weight of this insect is small enough to be carried by the forces resulting from the surface tension. The water surface acts like a film, and the pond-skater's feet form small indentations in the surface, causing the surface area to increase. This effect can also be observed by, very carefully, placing paper clips on the surface of a glass of water.

The question whether adding salt to water will change the surface tension of water seems to be a hotly debated subject. Many sources claim that there will be no change at all, some claim there will be a decrease and others still report an increase. This confusing state of affairs is probably the result of the fact that any change in the surface tension depends on the amount of salt added. In addition to this, the resulting effect even with just the right amount of salt is almost negligible. To put it in short, adding 10.5 g of certain inorganic salts to 100 ml of water can raise surface tension slightly.

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