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surface tension, capillarity and viscosity

Surface Tension:

  • The property of liquid by virtue of which it tries to minimize its free surface area is called surface tension.
  • Surface tension is the normal force acting per unit length on either side of an imaginary line drawn on the free surface of liquid and tangential to the free surface.
          Surface Tension (S) = F/l
Where F is the force acting on an imaginary line of length l
  • The minimum surface area of a given amount of liquid is for spherical shape that why rain drop are spherical in shape.
  • SI unit of surface tension is Newton per metre.
  • Surface tension decreases with the increase in temperature.
  • If the impurities are less soluble in liquid then its surface tension decreases.
  • If impurities are highly soluble in liquid then its surface tension increases.
Application of surface tension:
  • Surface tension decrease when soap, detergents or any other lubricants is mixed in water.
  • With the addition of salt in water its surface tension decreases.
  • When oil spreads over the surface of water, its tension decreases that’s why it is used for the prevention of mosquito from water.
  • Surface tension of mixture of soap or detergent is less than the pure water and that’s why soap cleans the cloths.
  • Antiseptic like Dettol have low surface tension and therefore it reaches in the tiny cracks of the wound and cleans the germs and bacteria.
  • Warm soup is tasty because at high temperature its surface tension is low and consequently the soup spread on all parts of the tongue.
  • Bubbles of soap solution are big because addition of soap decreases the surface tension of water.


  • The phenomena of rising or falling of liquid in a tube or column is called capillarity.
  • The height by which liquid ascends or descends in a capillarity tube depends upon the radius of the tube.
  • The capillarity depends on the nature of liquid and solid both.
  • The liquid which wets the wall of tube rises in the tube and the liquid which does not wet the walls of tube descends in the tube.
  • Water rises in the tube and form a concave meniscus while mercury descends in the tube form the convex meniscus.
Application of capillarity:
  • A piece of blotting paper soaks ink because the pores of the blotting paper serve as the capillary tubes.
  • The oil in the wick of the lamp rises due to the capillarity action of threads of in the wick.
  • Soil is ploughed and split into pieces by the farmers to prevent the loss of water, and it is due to capillarity action.
  • If a capillarity tube is dipped in water in an artificial satellite, water rises up to other end of the tube because of its zero apparent weight how long the tube may be.
  • Action of water soaking by the towel is due to capillarity action.
  • Melted wax rises up to the wick of the candle.


  • The force by virtue of which the relative motion between different layers of liquid or gases is opposed is called viscous force and this phenomenon is known as viscosity.
  • Viscosity is the property shown by both liquid and gases both.
  • The viscosity of liquid is due to cohesive force between its molecules.
  • The viscosity of a gas is due to diffusion of its molecules from one layer to other layers.
  • Viscosity in liquid is much greater in comparison to gases and an ideal liquid has zero viscosity.
  • There is no viscosity in solid.
  • Viscosity of a liquid decrease with the rise in temperature.
  • Viscosity of gases increase with the rise in temperature.
  • Viscosity is measure by its coefficient of viscosity and its SI unit is decapoise or pascal second.
Various terms related Viscosity:
Terminal Velocity:
  • When a body falls in a viscous solution its velocity first increases and finally becomes constant  and this constant velocity is known as terminal velocity.
  • The terminal velocity of a spherical body falling in a viscous medium is proportional to the square of radius of the body.
  • The radius of the spherical rain drops is very small therefore their terminal velocity is also small, with which they strike the earth’s surface.
  • When a liquid flow through a pipe its speed is maximum near axis and minimum near the walls of the pipe.
Streamline flow:
  • If a fluid is flowing in such a way that velocity of all the fluid particles reaching a particular point is same at all the time, then flow of liquid is said to be streamline flow.
  • That’s why in streamline flow each particle follows the same path as followed by a previous particle passing through that point.
Critical Velocity:
  • The maximum velocity up to which fluid motion is streamline is called critical velocity.
  • The flow upto critical velocity is streamline but above critical velocity flow is turbulent.
  • If the velocity of flow is more than the critical velocity the rate of flow depends on the density of fluid and not on viscosity, due to this reason on eruption  of the volcano the lava coming out of it flows very swiftly although it is very dense having large viscosity.

Bernoulli’s Theorem:

  • If a non-viscous and in-compressible liquid i.e. ideal fluid is flowing in a streamline flow then total energy i.e. sum of pressure energy and potential energy per unit volume of the liquid remains constant.
  • Venturi tube and aspirator pump works on Bernoulli’s theorem.
  • According to Bernoulli’s theorem with increase in velocity of liquid its pressure decreases and vice versa.