Surface Tension

Surface Tension

Table of Contents

Introduction to Surface Tension

  • Surface tension is a fascinating phenomenon in the realm of physics and fluid mechanics.
  • It refers to the property of a liquid that causes its surface to act like a stretched elastic membrane.
  • This property arises due to the cohesive forces between the liquid molecules.
  • At the molecular level, molecules inside a liquid are attracted to each other in all directions by cohesive forces.
  • However, molecules at the surface of the liquid experience a net inward force because they are surrounded by other molecules only on the sides and below, but not above the surface. This creates a tension or force at the surface, which tries to minimize the surface area of the liquid.

Definition of Surface Tension

Surface Tension (γ): It is the force acting perpendicularly on a unit length of the liquid’s surface.

Surface Tension Definition, Formula, Causes, Examples, And, 44% OFF



  • γ: Surface tension
  • F: Force acting perpendicular to the liquid surface
  • l: Length over which the force acts

Unit of Surface Tension

  • Surface tension is measured in Newton per meter (N/m).
  • Surface tension is measured in dyne per centimeter (dyn/cm).

Cohesive Forces:

  • At the surface of a liquid, molecules experience stronger cohesive forces due to fewer neighboring molecules.
  • This results in the formation of a “skin-like” layer on the surface, giving rise to surface tension.

Factors Affecting Surface Tension

Nature of Liquid

Surface tension varies based on the type of liquid.

Properties of Liquids - Course Hero

Cohesive Forces:

  • Different liquids have varying strengths of cohesive forces between their molecules.
  • Liquids with stronger cohesive forces tend to have higher surface tension.


  • Water has a higher surface tension compared to alcohols due to stronger hydrogen bonding.
  • Mercury exhibits very high surface tension due to strong metallic bonding.


Surface tension is temperature-dependent.

Does the surface tension of water increase or decrease with temperature? -  Quora

Effect of Temperature:

  • Increase in temperature generally leads to a decrease in surface tension.
  • Decrease in temperature leads to an increase in surface tension.


  • Cold water has higher surface tension than hot water.
  • Liquid metals like mercury show a significant drop in surface tension at elevated temperatures.

Impurities and Surfactants

  • Impurities: Foreign particles or substances present in the liquid.
  • Surfactants: Compounds that lower surface tension by disrupting cohesive forces.

Effect on Surface Tension:

  • Impurities can either increase or decrease surface tension, depending on their nature.
  • Surfactants generally reduce surface tension by weakening the cohesive forces between liquid molecules.

Practical Implications:

  • Detergents and soaps act as surfactants to reduce surface tension, aiding in cleaning processes.
  • Presence of oil or grease in water can alter its surface tension, affecting various industrial and environmental processes.

Capillary Action and Surface Tension

Capillary action refers to the ability of a liquid to flow against gravity in narrow spaces without the assistance of external forces like gravity.

  • Cohesive forces between liquid molecules and adhesive forces between liquid and solid surfaces drive capillary action.
  • In a capillary tube, these forces cause the liquid to rise or fall depending on the relative strengths of cohesive and adhesive forces.

Applications of Capillary Action


  • Capillary action helps in water absorption from the soil through the roots.
  • Enables transport of nutrients and water within the plant’s vascular system.

Paper Towels and Sponges:

  • Facilitates absorption of liquids due to the capillary spaces within their structure.
  • Useful in cleaning and wiping surfaces efficiently.

Ink Pens:

  • Uses capillary action to draw ink onto paper.

Blood Capillaries:

  • Capillary action plays a role in the circulation of blood in the human body.

Mathematical Representation of Capillary Rise



  • = Height of capillary rise
  • = Surface tension of the liquid
  • = Contact angle between the liquid and the capillary wall
  • = Radius of the capillary tube
  • = Density of the liquid
  • = Acceleration due to gravity

The formula quantifies the height to which a liquid will rise in a capillary tube due to surface tension and other factors like the contact angle, radius, density, and gravity.

Applications of Surface Tension

Liquid Droplets and Bubbles:

  • Liquid droplets and bubbles form due to surface tension pulling the liquid molecules together.
  • Surface tension provides the structural integrity that helps droplets and bubbles maintain their shape.
  • Raindrops, dew on leaves, and bubbles in soda are common examples where surface tension plays a crucial role.

Medical Applications:

  • Alveoli in the lungs remain open due to surface tension, facilitating efficient gas exchange.
  • Surfactants produced in the lungs reduce surface tension, preventing alveolar collapse and aiding in breathing.
  • Proper surface tension is vital for healthy respiratory function and preventing respiratory disorders.

Industrial Processes:

  • Surface tension controls the ejection of ink droplets onto paper, ensuring precise and high-quality printing.
  • Surface tension influences the flow and adhesion of paint on surfaces, affecting the finish and durability of paint jobs.
  • Surface tension determines the spreadability and uniformity of coatings, crucial for protective and decorative finishes.


In conclusion, surface tension is a fundamental concept in physics that describes the cohesive forces acting on the molecules at the surface of a liquid. It is defined as the force per unit length acting perpendicular to the liquid surface. Surface tension plays a crucial role in various natural phenomena and everyday applications, including the formation of liquid droplets, bubbles, and the phenomenon of capillary action.

Understanding the factors affecting surface tension, such as the nature of the liquid, temperature, and the presence of impurities, is essential for grasping its universal applicability in scientific, biological, and industrial contexts. The mathematical representation of surface tension and capillary rise provides quantitative insights into these phenomena, facilitating precise calculations and predictions.


Surface tension is the property of a liquid that allows its surface to behave like a thin elastic sheet. It’s caused by the attractive forces between the molecules of the liquid. These forces pull the molecules inward, minimizing the surface area of the liquid.

The SI unit of surface tension is newton per meter (N/m). It can also be expressed as joules per square meter (J/m²).

Surface tension (γ) is defined as the force (F) per unit length (L) of the line of force acting along the surface. Mathematically, it’s represented as:

γ = F / L

Imagine a layer of molecules on the surface of a liquid. Unlike the molecules in the bulk liquid, which are pulled in all directions by neighboring molecules, the surface molecules only experience attraction from below and the sides. This unbalanced pull creates a tension that tries to minimize the surface area.

Surface tension is responsible for many everyday phenomena:

  • Water striders walking on water: The legs of the water strider don’t break the water’s surface because surface tension creates a thin film that supports their weight.
  • Small insects on a pond: Tiny insects can rest on the water’s surface due to surface tension.
  • Curved water droplets: Water droplets form into a sphere because a sphere has the minimum surface area for a given volume.
  • Capillary action: Surface tension allows liquids to travel up narrow tubes against gravity. This is important in plants transporting water.


1. What is surface tension defined as?
a) Force per unit area at the surface of a liquid
b) Force per unit volume of a liquid
c) Mass per unit volume of a liquid
d) Velocity of a liquid

Answer: a) Force per unit area at the surface of a liquid

2. Which formula represents surface tension?

Answer: b)

3. What unit is used to measure surface tension in the SI system?
a) N/m
b) N
c) N/m^2
d) kg/m

Answer: a) N/m

4. Surface tension is due to:

a) Repulsive forces between liquid molecules
b) Attractive forces between liquid molecules
c) Neutral forces between liquid molecules
d) Electrical forces between liquid molecules

Answer: b) Attractive forces between liquid molecules

5. Surface tension ______ with the increase in temperature.
a) Increases
b) Decreases
c) Remains constant
d) Fluctuates randomly

Answer: b) Decreases

6. Which phenomenon is governed by surface tension in plants?
a) Transpiration
b) Photosynthesis
c) Capillary action
d) Respiration

Answer: c) Capillary action

7. What role does surface tension play in the formation of raindrops?
a) Increases the size of raindrops
b) Decreases the size of raindrops
c) Stabilizes the shape of raindrops
d) Changes the color of raindrops

Answer: c) Stabilizes the shape of raindrops

8. Which of the following reduces surface tension in the lungs?
a) Alveoli
b) Surfactants
c) Bronchi
d) Capillaries

Answer: b) Surfactants

9. What does the capillary rise formula include?


Answer: a) ℎ= r×ρ×g

10. Surface tension is responsible for the stability of:

a) Mountains
b) Valleys
c) Liquid droplets
d) Rocks

Answer: c) Liquid droplets

11. Which process uses surface tension in the separation of substances?
a) Distillation
b) Filtration
c) Chromatography
d) Decantation

Answer: c) Chromatography

12. In inkjet printing, surface tension controls the:

a) Color of the ink
b) Ejection of ink droplets
c) Size of the paper
d) Speed of printing

Answer: b) Ejection of ink droplets

13. What is the contact angle in the capillary rise formula?
a) Angle between liquid and solid
b) Angle between liquid and air
c) Angle between solid and air
d) Angle between liquid and container wall

Answer: a) Angle between liquid and solid

14. Which liquid will generally have a higher surface tension?
a) Water
b) Mercury
c) Alcohol
d) Oil

Answer: a) Water

15. Surface tension in liquids is due to _______ forces.
a) Tensile
b) Compressive
c) Shear
d) Cohesive

Answer: d) Cohesive

16. Surface tension is the reason behind the formation of:

a) Hurricanes
b) Tornadoes
c) Waves
d) Bubbles

Answer: d) Bubbles

17. What happens to the surface tension when soap is added to water?
a) Increases
b) Decreases
c) Remains the same
d) Becomes zero

Answer: b) Decreases

18. Which of these is not an application of surface tension?
a) Making soap bubbles
b) Cleaning using detergents
c) Climbing of water in plants
d) Cooking food

Answer: d) Cooking food

19. Surface tension helps in the formation of a convex or concave meniscus in a liquid?
a) Convex
b) Concave
c) Neither
d) Both

Answer: b) Concave

20. The stability of a liquid drop is due to its:

a) Volume
b) Mass
c) Surface tension
d) Temperature

Answer: c) Surface tension

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