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# Surface tension definition – Chemistry states of matter

## What is surface tension?

Surface tension is defined as the force in dynes acting perpendicularly to the tangential hypothetical lines to the surface of the liquid through 1 cm in length.

Surface tension is a characteristic property of liquids which depends upon the strength of intermolecular forces. It is denoted by ϒ, gamma or ó sigma. A molecule lying inside the liquid is attracted equally in all directions by the molecules around it. The molecules located in the free surface of the liquid are attracted only sideways and toward the interior.

The forces on the sides being counterbalanced, the surface molecule is pulled inward by the molecules in the bulk of the liquid. Thus, there is a tendency on the part of the surface molecules to go into the bulk of the liquid.

As a result, if this inward pull on all molecules lying at the surface, the surface behaves as if it were under tension. ( like a stretched membrane) and tends to counteract to the smallest possible area in order to have the minimum number of molecules at the surface. This contracting tendency of the free surface of a liquid is expressed in terms of surface tension.

Surface tension is defined as the force acting at right angles to the surface of the liquid along the one-centimeter length of the surface. Its unit is cm−1 or Newton per meter(NM-1).

Surface tension explains why the drop of liquid assumes a spherical shape because of surface tension reduces the area of the surface to a minimum. Hence sphere has a minimum surface area for a given volume of the liquid. Energy is required to increase the surface area in liquid. For example, consider a rectangular wire frame ABCD that is filled with liquid. In order to extend the surface area of the film, the movable wire has to be pulled from position CD to EF. Thus, some work has to be done against the force of surface tension.

The work required to be done to increase or extend the surface area by 1 sq. cm is called surface energy. The units of surface energy are ergs per sq.cm or joules per sq. meter.

In terms of dimensions,

surface energy = Work sq.cm

=(force*length) per sq. cm

=dynes*cm/ cm2 = dynescm2

Thus, the surface energy is same thing as surface tension.

## Application of Surface tension

(a) The spherical shape of liquid drops:

Due to the surface tension, liquid tends to contract to the smallest possible area in order to have the minimum number of molecules at the surface. Thus, the effect of surface tension is to reduce the area of the surface to the minimum. Hence, raindrops and mercury globules are spherical.

(b)Capillary action:

When a capillary tube is dipped into a liquid, their occurs either rise or fall in the liquid level in the capillary tube. The rise or fall of a liquid in capillary action. Capillary action is also due to surface tension.

For example, When a capillary tube is put into a liquid that wets glass (water), the liquid rises into the capillary tube to a certain height. This rise is due to the inward pull of surface tension acting on the surface which pushes the liquid into the capillary tube.

Similarly, when a capillary tube is put into a liquid that does not wet glass ( Mercury), the level inside the capillary falls below the level outside. Thus which wet the glass tube form the concave meniscus, while the liquids which do not wet the surface form convex surface in a capillary tube.

(c)Cleansing action of soaps and synthetic segments:

Soaps and detergents lower the interfacial tension between water and grease. Lowering of interfacial tension facilitates the mixing of water and grease.

(d)A shaving or a thin metallic needle floats on the surface of water due to surface tension.

Effect of Temperature on surface tension

The surface tension of liquids decreases with increases in temperature and become zero at the critical temperature( Where the meniscus between the liquid and the vapor disappears). Due to the disappearance of the free surface at the critical temperature become zero. When temperature increases, there is an increase in kinetic energy of liquid molecules hence intermolecular forces decreases. It results in a decrease in the inward pull functioning on the surface of the liquid.

In other words, surface tension decreases with increase in temperature

## Surface tension Examples

There are a lot of examples for the surface tension few of the examples are given below

• Floating of the needle of the in the surface of the water.
• Testing of jaundice with the help of surface tension it is possible because the normal surface tension of urine is 66 dynes/cm.
• soaps and Detergents they reduce the surface tension so that it gets easy to wash the clothes.
• surface tension for the Formation of droplets of water.