Definition of Surface Tension

It is the property of a liquid which enables it to resist tensile stress.

OR

It is the property of a liquid at rest by virtue of which a liquid surface tends to occupy a minimum surface area and behaves like stretched membrane. It is denoted by sigma(σ). Actually it is  the force per unit length of a liquid.

Unit of surface tension: The S.I unit of surface tension is N/m and C.G.S unit is dyne/cm.

Dimension of surface tension :  M1L0T-2

The surface tension of mercury at higher that of water at normal temperature.

definition of surface tension with example and unit

Applications and Example of Surface Tension

Rain drops and drops of mercury placed on glass plate are spherical due to this property.

Hair of shaving brush/painting brush, when dipped in waterspread out, but as soon as, it is taken out its hair stick together.

A greased needle placed gently on the free surface of water in a beaker does not sink due to surface tension.

Oil drop spreads on cold water but does not change shape on hot water.

Some value of important liquid:

Liquid Name Temperature in (ºC) SurfaceTension (mN/m, or dyn/cm)
Benzene 20 28.9
Carbon tetrachloride 20 26.8
Ethanol 20 22.3
Glycerin 20 63.1
Mercury 20 465.0
Olive oil 20 32.0
Soap solution 20 25.0
Water 0 75.6
Water 20 72.8
Water 60 66.2
Water 100 58.9
Oxygen -193 15.7
Neon -247 5.15
Helium -269 0.12

What is Capillarity? :

It is defined as a phenomenon of rise or fall of a liquid surface in small vertical tube held in a liquid relative to general level of the liquid. Capillary action is due to both cohesion and adhesion.

The height of rise or fall (h) in the tube is given by,

h= 4σcosα/wd

where,

σ= surface tension

α= Angle of contact of the liquid surface,

w= specific weight of liquid, and

d= diameter of the capillary tube.

It may be noted that if cohesion force is more than that of adhesion force, then shape of liquid at free surface in capillary tube is concave.

Example  – mercury in capillary tube.

If cohesion force is less than that of adhesion force, then shape of liquid at free surface in capillary tube is convex.

Example – water in capillary tube.

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