Surface Tension

States of Matter of Class 11

Surface Tension

Surface Tension of a liquid is defined as the force acting at right angles to the surface along one centimeter length of the surface. It is represented by the greek letter gamma,γ.

Due to surface tension molecules tend to leave the surface, i.e. the surface of the liquid tends to contract to the smallest possible area for a given volume of the liquid. Further for a given volume of the liquid, sphere has the minimum surface area. This explains why the drops of a liquid are spherical.

Thus it is apparent that in order to increase its surface area, force must be exerted to overcome the surface tension. In other words, work will have to be done to increase the surface area. Thus the surface tension of a liquid is defined as the work (energy) required to expand the surface of a liquid by unit area. Mathematically,

surface tension = Work done/Change in area

Thus surface tension of a liquid may also be defined as the force in dynes necessary to rupture its surface along one centimeter length. In SI units it is defined as the force in newtons (1 newton = 105 dynes)required to rupture  newton 1 meter length of the surface of a liquid.

Thus the units of surface tension are dynes per cm (or Newtons per metre, Nm-1 in SI system).

Surface tension of a liquid is measured with the help of apparatus called stalgmometer.

The surface tension of a liquid decreases with increase of temperature and becomes zero at its critical temperature (where the surface of separation between liquid and its vapour disappears). The decrease in surface tension with increase of temperature is due to the fact that with increase of temperature, the kinetic energy of the molecule (and hence the speed of molecules) increases and hence the intermolecular forces of attraction decreases.

Surface tension in everyday life

  • Cleansing action of soap and detergents is due to their property of lowering the interfacial tension between water and greasy substances. Thus soap solution due to its lower surface tension can penetrate into the fibre to surround the greasy substances and wash them away.

  • Efficacy of tooth pastes, mouth washes and nasal jellies is partly due to the presence of substances having lower surface tension. Lowering of interfacial tension helps these preparations to spread evenly over the surface with which they come in contact thereby increasing the efficiency of their antiseptic action.

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