Surface Tension Made Simple

Surface Tension Made Simple

1. Cohesive Force

• The force of attraction between the molecules of the same substance is called the cohesive force.

• Cohesive force is maximum in solids. 

• This is why solids have a fixed shape.

• Cohesive force is negligible in the case of gases.

2. Adhesive Force

• The force of attraction between the molecules of different substances is called the adhesive force.

• Due to adhesive force, one body sticks to the other.

3. Surface Tension

• Surface tension is the property of a liquid by virtue of which it tends to have the area of its free surface minimum as if it were under tension like a stretched elastic membrane.

Spherical Shape of a Liquid Drop

A liquid drop attains a spherical shape due to surface tension, since, for a given volume, a sphere has the minimum surface area.

Measurement

• 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 the liquid and tangential to the free surface.

So, if a force F acts on an imaginary line of length l, then surface tension,

T = F/l

Work Done

• Work done in increasing the surface area of a liquid by unity under isothermal conditions is equal to the surface tension of the liquid.

• According to this definition, the unit of surface tension is joule/meter².

Temperature Dependence

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

4. Capillary Tube

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

5. Capillarity

• If a capillary tube is dipped in a liquid, the liquid ascends or descends in the capillary tube.

• This phenomenon is called capillarity.

Factors Affecting Capillarity

The height by which liquid ascends or descends in a capillary tube depends upon the radius of the tube. The capillarity depends on the nature of both the liquid and the solid.

• The liquid that wets the wall of the tube rises in the tube.

• The liquid that does not wet the wall of the tube descends in the tube.

Examples:

• When a glass capillary tube is dipped in water, water rises in the tube, and the shape of the water meniscus is concave.

• When a glass capillary tube is dipped in mercury, mercury descends in the tube, and the shape of the mercury meniscus is convex.

Illustrations of Capillarity

1. A piece of blotting paper soaks ink because the pores of the blotting paper serve as capillary tubes.

2. The oil in the wick of a lamp rises due to the capillary action of the threads in the wick.

3. The root hairs of plants draw water from the soil through capillary action.

4. To prevent loss of water due to capillary action, the soil is loosened and split into pieces by the farmers.

5. If a capillary tube is dipped in water in an artificial satellite, water rises up to the other end of the tube because of its zero apparent weight, however long the tube may be.

6. The action of a towel in soaking up water from the body is due to the capillary action of cotton in the towel.

7. Melted wax in a candle rises up to the wick by capillary action.

6. Surface Tension Applications

• If a clean and dry needle is kept slowly on the surface of water, it floats due to surface tension.

• The addition of detergent or soap decreases the surface tension of water and thus increases the cleaning ability.

• Bubbles of soap solution are big because the addition of soap decreases the surface tension of water.

• When kerosene oil is sprinkled on water, its surface tension decreases. As a result, the larva of mosquitoes floating on the surface of water die due to sinking.

• Warm soup is tasty because at high temperature its surface tension is low, and consequently the soup spreads on all parts of the tongue.

• If the liquid surface is curved, there is always an excess pressure on the concave side.

Formulas

• Excess pressure in a liquid drop =

• Excess pressure in a soap bubble =

T = surface tension, R = radius

Final Thoughts

Cohesive force is the attraction between molecules of the same substance, making solids retain a fixed shape, while gases barely show this force. Adhesive force, on the other hand, helps different substances stick together. Surface tension gives liquids a natural “skin,” making drops spherical and allowing objects like needles to float on water.

It also decreases with temperature and can be reduced by soap or detergent, which improves cleaning. Capillarity occurs when liquids rise or fall in narrow tubes, depending on whether they wet the surface, and it plays a role in plants drawing water from the soil or ink soaking into blotting paper.

Everyday examples like oil rising in lamp wicks, water absorption in towels, and wax moving up candle wicks show how these forces shape the world around us. Even the taste of warm soup is influenced by surface tension, as it spreads across the tongue more easily.