Short Answer

Factors that affect surface tension include temperature, impurities, nature of the liquid, presence of dissolved substances, and surface cleanliness. When temperature increases, surface tension decreases. Adding impurities like soap or detergent also reduces surface tension.

The natural properties of the liquid and the type of dissolved substances can either increase or decrease surface tension. Dust or oil on the surface can weaken the surface layer of the liquid. These factors decide how strongly the surface molecules attract each other.

Detailed Explanation :

Factors affecting surface tension

Surface tension is the property of a liquid that makes its surface behave like a stretched elastic sheet. It is caused by cohesive forces between the molecules of the liquid. However, this property does not remain the same under all conditions. Several factors can increase or decrease surface tension by affecting the strength of cohesion among liquid molecules.

Understanding these factors helps us explain various behaviours of liquids in daily life, laboratory experiments, industry, and nature.

Below are the main factors that affect surface tension.

1. Temperature

Temperature has one of the strongest effects on surface tension.

• As temperature increases, surface tension decreases.
• As temperature decreases, surface tension increases.

This happens because at higher temperature:

• Molecules gain more kinetic energy.
• They move faster and break away from each other easily.
• Cohesive forces become weaker.

For example:

• Hot water has lower surface tension than cold water.
• Soap bubbles form better with warm water because of lower surface tension.

This is why liquids spread faster on surfaces when they are warm.

2. Impurities (especially soaps and detergents)

Adding impurities changes the way molecules interact at the surface.

• Soaps and detergents reduce surface tension by weakening the forces between water molecules.
• This is why detergents help water spread and clean surfaces more effectively.

Other impurities may increase surface tension, depending on how they interact with the liquid.

For example:

• Oil added to water reduces surface tension.
• Some organic compounds increase surface tension slightly.

Impurities can either strengthen or weaken the surface, depending on their chemical nature.

3. Nature of the liquid

Different liquids naturally have different surface tensions because of the strength of their molecular forces.

• Water has high surface tension due to strong hydrogen bonding.
• Alcohol has low surface tension because its molecules attract each other weakly.
• Mercury has extremely high surface tension because of strong metallic bonding.

Thus, the internal bonding structure of a liquid decides its natural surface tension.

4. Presence of dissolved salts

Dissolved salts usually increase surface tension.

For example:

• Adding table salt (sodium chloride) to water increases surface tension.
• Other ionic salts like magnesium chloride or calcium chloride also increase it.

This happens because dissolved ions strengthen the attraction between water molecules, making the surface tighter.

However, some substances, especially organic molecules, may lower surface tension.

5. Surface cleanliness

The cleanliness of the liquid surface also affects surface tension.

• If the surface has dust, oil, or grease, surface tension decreases.
• A clean and pure surface has higher surface tension.

For example:

• A needle floats on clean water due to high surface tension.
• It sinks easily in oily or dirty water because surface tension is reduced.

Surface cleanliness is important in scientific experiments, as even small amounts of impurities can affect results.

6. Electrical charge at the surface

Surface tension can be affected by electric charge:

• Adding ions (charged particles) can increase or decrease surface tension.
• Applying an external electric field can also change surface properties.

This effect is important in industries like inkjet printing and electro-coating.

7. Surfactants

Surfactants are special chemicals that reduce surface tension dramatically.

Examples include:

• Detergents
• Soaps
• Shampoos

Surfactants collect at the liquid surface and weaken the cohesive forces, allowing water to spread wider and penetrate more easily. They are used in cleaning, painting, spraying, and lubrication.

8. Atmospheric pressure

Atmospheric pressure has a small effect on surface tension:

• Higher pressure slightly increases surface tension.
• Lower pressure slightly decreases it.

Though this effect is minor, it becomes important in scientific experiments and high-altitude studies.

Importance of understanding these factors

Knowing the factors that affect surface tension helps in:

• Efficient cleaning and washing
• Forming bubbles and foams
• Understanding raindrop formation
• Designing sprays, inks, and paints
• Studying insect movement on water
• Improving industrial processes like coating and polishing

Everyday activities such as cooking, cleaning, and simple experiments also depend on these factors.

Conclusion

Surface tension depends on many factors, including temperature, impurities, nature of the liquid, dissolved salts, and surface cleanliness. Higher temperature and impurities like soaps reduce surface tension, while clean surfaces and dissolved salts often increase it. Understanding these factors helps explain how liquids behave in daily life, nature, and industrial applications. It also helps in designing better cleaning methods, sprays, coatings, and scientific experiments.