Introduction to surface tension of water
Surface tension of water is the property of water at rest by virtue of which its surface behaves like a stretched membrane and tries to occupy minimum possible surface area.
Mathematically, it is the force per unit length of an imaginary line drawn in the plane of water surface acting right angles to this line.
For any liquid,
Surface tension, T = F/l (where F is the force acing on the imaginary line of length l )
Value of surface tension of water in SI is 0.072 Nm-1 and in CGS is 72 dynes cm-1.
It means that it takes 0.072 Newton force to break the 1m long surface film of water .
This value of surface tension of water is higher than other liquids except the surface tension of mercury (0.425 Nm-1 ). Surface Tension force acts in all directions parallel (i.e. tangential) to the water surface. Therefore, the free surface of water behaves like an elastic skin on a state of tension.
Origin of Surface Tension of Water
In the interior of water, a molecule is equally surrounded by other molecules and thus it is equally attracted by other molecules. Hence the molecules experience the force of attraction equally in all directions and the resultant force is zero.
But the molecules in the interior of water surface is attracted by other molecules of surface and molecules below the surface. Thus they experience a net force which pulls molecules towards the interior of the liquid. Due to this, surface water behaves as a stretched membrane and a kind of stress develops. This creates the surface tension in water surface.
Factors Affecting Surface Tension of Water
- Temperature: Increase in temperature increases the kinetic energy of water molecules and decreases the intermolecular force of attraction. This decreases the surface tension. So, it is more efficient to wash clothes in hot water than in cold water.
- Presence of impurities: Surface tension of liquid depends upon the nature of impurities present in solution.
- Those impurities which concentrate on the surface of liquid than bulk are surface active agent or surfactant. The surfactant reagents lower the surface tension. E.g. soap, detergent etc.
- Those impurities which concentrate on the bulk rather than on surface are bulk active reagents.These reagents increase the surface tension. E.g. NaCl (Sodium Chloride), K2SO4
Due to this reason, salt dissolved water has more surface tension than that of soap water and mosquitoes sink easily in soap water.