Lesson 7, Topic 1
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# Surface Tension

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When a bucket is filled with water and left for a few days in an open place, it will be observed that the surface of the water will be covered with particles of a stretched skin, likewise, when water is dropping slowly from a tap, it first forms a bubble before bursting.

Also, a needle rubbed with oil or grease can be made to float on the surface of water if placed gently and the oil will form a film on the surface of the water like a stretched elastic skin showing that there is a force or tension along the surface of the water and this force is referred to as surface tension.

Surface tension is the ratio of tangential force, F, in the surface of a liquid to the length along which the force acts. It is the force that acts along the surface of a liquid and causes the liquid surface to behave like a stretched elastic skin.
OR
Surface tension is a phenomenon in which the surface of a liquid behaves/acts as if it is covered by an elastic skin.

Therefore,

⇒ $$\scriptsize Surface\: tension = \normalsize \frac{Force}{Length}\\ = \normalsize \frac{F}{L}$$

And its unit is Newton per metre (Nm-1).

### Molecular Explanation of Surface Tension:

Surface tension is due to the intermolecular forces of attraction between molecules of the liquid. Consider a liquid in a container with two molecules A and B in a mass of the liquid as shown.

Molecules of liquid anywhere in the liquid (e.g molecule A) experience an equal amount of molecular attraction by the neighbouring molecules and therefore have zero resultant force of attraction.

Molecules of liquid at the surface of the liquid, like molecule B, only experience molecular force of attraction from molecules of liquid below the liquid surface. As a result, a force (F) develops that is directed at the interior of the medium, tends to shrink the liquid surface, reduce the surface area of the liquid and strain the liquid surface. Due to the unbalanced force, the free surface behaves like a thin film under tension.

Water molecules have a relatively higher force of attraction between them due to hydrogen bonds. That is why water has higher surface tension than most other liquids.

### Some Applications of Surface Tension:

1. In canopies and umbrellas: Canopies and umbrellas including raincoats are treated with oil which forms a thin film on the fabrics when rainwater falls on them. These films prevent water from seeping through the thread holes provided the oil film is not broken.

2. Cleaning Action:  The use of detergents helps weaken the tension on the surface of water and assists in floating away the dirt particles or oil. Hot temperatures also assist in decreasing the surface tension of water.

3. Toothpaste spreads more freely in the mouth while cleaning the mouth because it contains soap that reduces surface tension.

4. Mosquito eggs float on the surface of the water. Kerosene is sprayed on the surface of the water to reduce surface tension which causes the egg to go inside the water, hence, stopping the breeding of mosquitoes.

### Reduction of Surface Tension:

Surface tension can be reduced by:

1. Adding impurities such as detergents, and soap.
2. Increasing the temperature of the liquid.
3. By the addition of alcohol, camphor.

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