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# SURFACE TENSION

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How does surface tension act? What is basic cause?
Answer: Surface tension is the property of a liquid by virtue of which its free surface behaves like a stretched elastic membrane.
When a blade is carefully kept on a liquid surface, it floats; but it sinks if it is dropped carelessly.

A Shaving Brush has its hairs spread when dry, but when dipped in water and taken out, the hairs cling down.

A drop of water assumes spherical shape because of surface tension as the surface area is minimum for spherical shape for a given volume of it.

From these demonstrations we can conclude that the free surface of a liquid always tries to minimize the surface area.

The cause of Surface Tension

The cohesive forces among the liquid molecules are responsible for this phenomenon of surface tension. Well inside the liquid, each molecule is pulled equally in every direction by neighboring liquid molecules, resulting in a net force of zero. The molecules at the surface do not have other molecules on all sides of them and therefore are pulled inwards. This creates some internal pressure and forces liquid surfaces to contract to the minimal area.

Surface tension of a liquid is measured by the force acting per unit length on either side of an imaginary line drawn on the free surface of liquid, the direction of this force being perpendicular to the line and tangential to the free surface of liquid. So if F is the force acting on one side of imaginary line of length L, then T = (F/L)

Observations based on Surface Tension

If a small irregular piece of camphor is floated on the surface of pure water, it does not remain steady but dances about on the surface. This is because, irregular shaped camphor dissolves unequally and decreases the surface tension of the water locally. The unbalanced forces make it move haphazardly in different directions.

Factors Affecting Surface Tension

Temperature : The surface tension of liquid decreases with rise of temperature. The surface tension of liquid is zero at its boiling point and it vanishes at critical temperature. At critical temperature, intermolecular forces for liquid and gases becomes equal and liquid can expand without any restriction.

Impurities : The presence of impurities either on the liquid surface or dissolved in it, considerably affect the force of surface tension, depending upon the degree of contamination. A highly soluble substance like sodium chloride when dissolved in water, increases the surface tension of water. But the sparingly soluble substances like phenol when dissolved in water, decreases the surface tension of water.

See uncommon applications of surface tension in Comments

1. Clinical test for jaundice

Normal urine has a surface tension of about 66 dynes/cm but if bile is present (a test for jaundice), it drops to about 55. In the Hay test, powdered sulfur is sprinkled on the urine surface. It will float on normal urine, but sink if the S.T. is lowered by the bile.

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