Short Answer

Surface tension is the force that acts on the surface of a liquid and makes the surface behave like a stretched elastic sheet. It is caused by the strong attraction between liquid molecules at the surface. Because of surface tension, small insects can walk on water and drops of water form a round shape.

The molecules inside a liquid are pulled equally in all directions, but surface molecules are pulled inward more strongly. This inward pull creates tension on the surface, making it tight and stable. Surface tension depends on the nature of the liquid and temperature.

Detailed Explanation

Surface Tension

Surface tension is a special property of liquids that makes their surface behave as if it is covered with a thin, stretched film. It is created because the molecules on the surface of a liquid experience a stronger inward force than those inside the liquid. This inward pulling force makes the surface of the liquid tight and resistant to being broken.

Surface tension is responsible for many everyday phenomena such as water droplets forming round shapes, insects walking on water, and needles floating on the surface of water when placed carefully.

Why Surface Tension Occurs

To understand surface tension, we must look at how liquid molecules behave.

• Inside the liquid, each molecule is surrounded by other molecules. These molecular forces balance out in all directions.
• On the surface, molecules are not surrounded on all sides. They experience a stronger pull from inside the liquid.
• This inward pull creates tension on the surface, making it behave like a stretched elastic sheet.

The stronger the attraction between molecules, the greater the surface tension.

Molecular Explanation of Surface Tension

Surface tension is closely linked to intermolecular forces. The molecules at the surface have higher potential energy because they lack the same number of neighbors as interior molecules. To minimize this energy, the surface area becomes as small as possible.

• Liquids try to minimize their surface area due to surface tension.
• This is why water forms spherical droplets. A sphere has the smallest surface area for a given volume.

Water has strong hydrogen bonding, so it has high surface tension. Liquids like alcohol or petrol have weak attractions, so they have low surface tension.

Factors Affecting Surface Tension

Surface tension does not remain constant. Several factors influence it:

1. Temperature

Temperature has a major effect on surface tension.

• Increase in temperature → decrease in surface tension
  Molecules move faster and break the surface bonds more easily.
• Decrease in temperature → increase in surface tension
  Molecules move slower and remain more tightly bound.

This is why warm soap water spreads easily, while cold water holds together more strongly.

2. Nature of the Liquid

Different liquids have different strengths of intermolecular forces.

• Strong forces → high surface tension
  Example: water, mercury
• Weak forces → low surface tension
  Example: alcohol, oil, petrol

Water has high surface tension due to hydrogen bonding.
Mercury has even higher surface tension due to metallic bonding.

3. Impurities

Impurities affect surface tension in different ways.

• Surface-active impurities (like soap or detergents) lower surface tension.
  This is why soap allows water to spread and clean better.
• Other impurities may increase surface tension depending on their chemical nature.

4. Presence of Surfactants

Surfactants (surface-active agents) reduce surface tension drastically.
Example: detergents, shampoos, liquid soaps.

They reduce the attraction between molecules at the surface, making the liquid spread out easily.

5. Air and Environmental Conditions

Humidity, dust, or dissolved gases may slightly affect surface tension.

Examples of Surface Tension in Daily Life

Surface tension is seen everywhere around us:

1. Water Droplets

Drops of water are round because surface tension pulls the surface inward.

2. Insects Walking on Water

Some insects like water striders walk on water due to high surface tension.

3. Floating Needle

A steel needle can float on water if placed gently because surface tension holds it.

4. Soap Bubbles

Soapy water has low surface tension, allowing bubbles to form easily.

5. Capillary Action

Surface tension helps water rise in thin tubes, important in plants for water transport.

6. Raindrops on Leaves

Drops remain rounded because the surface tension keeps them contracted.

Importance of Surface Tension

Surface tension plays a key role in:

• Biological systems
• Cleaning action of detergents
• Ink spreading in pens
• Paint application
• Functioning of lungs
• Movement of water in plants
• Droplet formation in clouds

It is important in both natural and industrial processes.

Conclusion

Surface tension is the force that makes the surface of a liquid act like a stretched film due to the strong attraction between surface molecules. It explains many natural and everyday phenomena such as water droplets, insect movement on water, and bubble formation. Surface tension depends on temperature, intermolecular forces, and impurities. Understanding this property helps explain how liquids behave at their surfaces in various scientific and practical situations.