Surface Tension

Cohesive Force: The force of attraction between the molecules of same substance is called cohesive force. Cohesive force is maximum in solids. This is why solids have a fixed shape. Cohesive force is negligible in case of gases.

Adhesive Force: Force of attraction between the molecules of doddering substances is called adhesive force. Due to adhesive force, one body sticks to other.

Surface Tension: Surface tension is the property of a liquid by virtue of which it has the tendency to have the areas of its free surface minimum as if it were under tension like a stretched elastic membrane.

Surface tension of a liquid is measured by 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.

So, if a force F acts on an imaginary line of length I, then surface tension, T = F/ I .

Work done in increasing the surface area of a liquid by unity under isothermal condition is equal to surface tension of liquid. According to this definition, unit of surface tension is joule/ meter².

Surface tension of a liquid decreases with the increase of temperature and becomes zero at critical temperature.

Capillary tube: A tube having very narrow (fine) and uniform bore is called a capillary tube.

Capillarity: if a capillary tube is dipped in a liquid, liquid ascent or descends in the capillary tube. This phenomenon is called capillarity.

The height by which liquid ascends or depends in a capillary tube spends upon the radius of 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 wall of tube descends in the tube. For example, when a glass capillary tube is dipped in water, water rises in the tube and shape of water meniscus is concave, similarly when a glass capillary tube is dipped in mercury, mercury decents in the tube and shape of mercury meniscus is convex.