Short Answer

Surface tension is the property of a liquid that makes its surface behave like a stretched elastic sheet. It happens because the molecules at the surface of a liquid are pulled inward by strong attractive forces. As a result, the surface becomes tight and can even support light objects like a needle or small insects.

In simple words, surface tension is the force that makes liquid surfaces contract and form droplets. It allows water to form beads on a leaf and makes soap bubbles possible. Surface tension is strongest in liquids with strong intermolecular forces.

Detailed Explanation :

Surface tension

Surface tension is an important property of liquids that explains why their surfaces behave in a special way. It is defined as the force acting on the surface of a liquid that tends to minimize its surface area. In simple terms, it makes the liquid surface act like a thin, stretched, and flexible membrane. This force is caused by the cohesive forces (attractive forces) between the molecules of the liquid.

To understand surface tension, imagine water in a glass. The molecules inside the water are surrounded by other molecules on all sides. These molecules attract each other equally. But the molecules at the surface do not have other molecules above them, so they are pulled strongly inward and sideways by the molecules below. This inward pull creates tension on the surface, making it tight. This property is what we call surface tension.

Surface tension is responsible for many natural and everyday phenomena. It allows insects to walk on water, creates water droplets, and helps soap bubbles form. It also plays a role in capillary action, where water rises in narrow tubes.

1. Cause of surface tension: Cohesive forces

Surface tension is mainly caused by cohesive forces. These are the attractive forces between molecules of the same substance. Water molecules attract each other through strong hydrogen bonds. These forces pull the surface molecules tightly together, reducing the surface area.

Liquids with stronger cohesive forces have higher surface tension. For example:

• Water has high surface tension.
• Alcohol has low surface tension because its molecules have weaker attractions.

2. Behavior of molecules at the surface

Molecules at the surface of a liquid behave differently from molecules inside the liquid.

• Inside the liquid, molecules are surrounded on all sides, so the attractive forces balance out.
• On the surface, molecules are only pulled inward and sideways, not upward.
  This imbalance of forces creates tension on the surface.

This tension tries to keep the surface area as small as possible. As a result, water tends to form spherical droplets because a sphere has the smallest surface area for a given volume.

3. Examples of surface tension in daily life
4. Water droplets

Water forms round droplets because surface tension pulls the water molecules inward, making the surface contract into a sphere.

1. Insects walking on water

Small insects like water striders can walk on water due to high surface tension. The tight surface acts like a flexible skin that can support their weight.

1. Floating of a needle

A steel needle can float on water even though it is heavier than water. It floats because surface tension supports it as long as the surface is not disturbed.

1. Soap bubbles

Soap reduces water’s surface tension, allowing the water film to stretch and form bubbles. Without lowering surface tension, bubbles would not form easily.

1. Capillary action

Surface tension helps liquids climb up thin tubes. This is important in plants, where water moves upward through narrow vessels.

4. Effect of temperature on surface tension

Surface tension decreases with an increase in temperature. When a liquid is heated:

• Molecules move faster
• Cohesive forces become weaker
• Surface tension decreases

That is why hot water forms weaker droplets than cold water.

5. Effect of impurities

Impurities can either increase or decrease surface tension depending on the type of impurity.

• Soaps and detergents decrease surface tension by breaking the hydrogen bonds.
• Some dissolved salts increase surface tension.

This is why detergents make water spread out easily on clothes for better cleaning.

6. Effect of intermolecular forces

Liquids with strong intermolecular forces have high surface tension. Water has high surface tension because of strong hydrogen bonding. Alcohol and oils have low surface tension because their molecules do not attract each other strongly.

7. Importance of surface tension

Surface tension is important in many natural and scientific processes:

• Helps insects move on water
• Helps plants absorb water through roots
• Useful in painting, cleaning, and ink spreading
• Helps blood move in small capillaries
• Important in formation of raindrops

Thus, surface tension plays a vital role in both nature and technology.

Conclusion

Surface tension is the force that makes the surface of a liquid act like a stretched elastic sheet. It is caused by the cohesive forces between molecules. Many daily activities and natural processes—such as droplet formation, insect movement on water, floating needles, and capillary action—are due to surface tension. Temperature, impurities, and intermolecular forces can affect the strength of surface tension. It is a very important concept in understanding the behavior of liquids.