Short Answer

Surface tension is affected mainly by temperature, intermolecular forces, and the presence of impurities or surfactants. Liquids with strong intermolecular forces, like water, have high surface tension, while liquids with weak forces, like alcohol, have low surface tension.

Temperature also changes surface tension. When temperature increases, surface tension decreases because molecules move faster and break the surface bonds more easily. Impurities such as soap reduce surface tension by weakening the attraction between surface molecules.

Detailed Explanation

Factors Affecting Surface Tension

Surface tension is the force that makes the surface of a liquid behave like a stretched elastic sheet. It arises due to the attractive forces between the molecules at the surface. These forces pull the surface molecules inward, creating tension. Surface tension varies from liquid to liquid and changes with conditions like temperature, impurities, and the nature of the liquid.

Understanding the factors that affect surface tension is important in chemistry, biology, food science, and many industrial applications such as detergents, paints, ink spreading, and droplet formation.

Below are the major factors that influence surface tension.

1. Temperature

Temperature has a strong effect on surface tension.

• Increase in temperature → Decrease in surface tension
  As temperature rises, molecules gain kinetic energy, move faster, and break free from the surface more easily. The cohesive forces weaken, reducing surface tension.
• Decrease in temperature → Increase in surface tension
  At low temperatures, molecules move slowly, so attractive forces are stronger, increasing surface tension.

Examples:

• Hot water forms flatter drops than cold water because its surface tension is lower.
• Soap bubbles cannot form well in very hot water because surface tension becomes too low.

2. Nature of the Liquid (Intermolecular Forces)

Different liquids have different strengths of attraction between molecules.

• Strong intermolecular forces → High surface tension
  Examples: water, mercury
  Water has strong hydrogen bonding, while mercury has metallic bonding, giving it very high surface tension.
• Weak intermolecular forces → Low surface tension
  Examples: alcohol, benzene, petrol
  These liquids spread easily and do not form strong droplets.

Thus, the type and strength of intermolecular forces play a major role in determining surface tension.

3. Impurities on the Surface

Impurities may either increase or decrease surface tension depending on their chemical nature.

• Impurities that reduce attraction (surfactants) lower surface tension.
  Examples: soap, detergent, shampoo.
  These substances spread on the surface and weaken molecular attraction.
• Impurities that increase attraction may raise surface tension.
  Some solutes strengthen bonding depending on their composition.

Example:
Adding salt to water slightly increases its surface tension.

4. Presence of Surfactants

Surfactants (surface-active agents) are chemicals specially designed to reduce surface tension.

How surfactants work:

• They position themselves at the surface
• Reduce the cohesive forces
• Allow the liquid to spread out more easily

This is why soap helps water clean surfaces, wash clothes, and remove grease—it reduces surface tension and allows water to spread and penetrate.

Examples of surfactants:

• Soap
• Detergents
• Liquid wash
• Shampoos

5. Dissolved Substances

Different dissolved substances affect surface tension differently:

• Soluble salts (e.g., NaCl) increase surface tension because they strengthen attraction between molecules.
• Organic substances (e.g., alcohol) decrease surface tension because they weaken molecular attraction.

This explains why adding alcohol to water lowers its surface tension.

6. Environmental Conditions

Surface tension may also be influenced by:

• Humidity
• Dust particles
• Dissolved gases
• Atmospheric pressure

Although these effects are small, they can slightly shift surface tension values.

Practical Examples Showing Effects on Surface Tension

Walking Insects

Insects like water striders walk on water because water has high surface tension. Soap reduces surface tension and the insects sink.

Floating Needle

A needle floats on water due to high surface tension. When soap is added, surface tension drops and the needle sinks.

Soap Bubbles

Surface tension must be low for bubbles to form. Soap lowers surface tension so bubbles can stretch without breaking.

Droplet Formation

Water forms round droplets because surface tension pulls the surface inward. Alcohol spreads out because its surface tension is low.

Washing Clothes

Detergents reduce water’s surface tension, helping water spread and remove dirt.

These examples show how important surface tension is in daily life and how these factors affect it.

Conclusion

Surface tension is influenced by temperature, the strength of intermolecular forces, impurities, surfactants, and dissolved substances. Higher temperature and surfactants reduce surface tension, while strong intermolecular forces increase it. Understanding these factors helps explain everyday phenomena like bubble formation, droplet behavior, washing action, and the movement of insects on water. Surface tension is a key concept in chemistry and plays an important role in many natural and industrial processes.