Surface Tension Explained
Surface tension is the property of liquids whereby they behave as if covered by a thin elastic skin. This occurs because molecules at the liquid surface ex
TL;DR: Surface tension is the property of liquids whereby they behave as if covered by a thin elastic skin. This occurs because molecules at the liquid surfa…
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
Surface Tension: Surface tension is the property of liquids whereby they behave as if covered by a thin elastic skin. This occurs because molecules at the liquid surface ex
Surface Tension Explained Simply
Surface tension is the property of liquids whereby they behave as if covered by a thin elastic skin. This occurs because molecules at the liquid surface ex This is a key Physics concept for Class 11.
What is Surface Tension?
Surface tension is the force acting per unit length along the surface of a liquid, measured in N/m or dyne/cm. It arises because liquid molecules at the surface experience net inward forces from bulk liquid, while surface molecules experience no compensating force from above. To overcome this imbalance, molecules at the surface arrange themselves to minimize surface area. Surface tension acts like an elastic membrane covering the liquid. The property explains water droplet formation (spheres minimize surface area), water beading on leaves, and ability of some insects to walk on water.
Molecular Explanation
Liquids are held together by intermolecular forces (hydrogen bonds for water, van der Waals forces for others). For molecules deep inside liquid, attractive forces from neighboring molecules are balanced in all directions. For surface molecules, there are attractive forces only from the liquid side, creating net inward force. This imbalance causes surface molecules to be pulled inward, trying to minimize surface area and the number of exposed molecules. The stronger the intermolecular forces, the higher the surface tension. Water has unusually high surface tension due to strong hydrogen bonds.
Factors Affecting Surface Tension
Temperature: Surface tension decreases with increasing temperature. Higher temperature increases molecular motion, reducing intermolecular attractions. Nature of liquid: Different liquids have different intermolecular forces, thus different surface tensions. Water has very high surface tension (~72 mN/m at 20°C), while mercury has even higher (~487 mN/m). Impurities: Dissolved substances affect surface tension. Soaps and detergents significantly reduce water's surface tension, which aids in cleaning. Adding salt to water increases surface tension slightly.
Measurement of Surface Tension
Surface tension can be measured using several methods: Capillary rise method - liquid rises in narrow tube; height depends on surface tension. Stalagmometer - counts drops; weight per drop relates to surface tension. Force method - measures force needed to pull wire from liquid surface. Sessile drop method - analyzes shape of hanging drop. The capillary rise method is most common in school labs. The formula h = 2T cosθ / (ρgr) relates height to surface tension, where T is surface tension, θ is contact angle, ρ is liquid density, g is gravity, and r is tube radius.
Capillarity
Capillarity is the flow of liquid in narrow tubes due to surface tension and adhesive forces between liquid and tube. If adhesive forces (liquid-glass) exceed cohesive forces (liquid-liquid), the liquid wets the glass and rises in the capillary (water in glass). If cohesive forces exceed adhesive forces, the liquid does not wet the glass and is depressed (mercury in glass). The height reached is given by h = 2T cosθ / (ρgr). Plants use capillarity to draw water from roots upward through xylem vessels. Blotting paper absorbs water due to capillarity.
Practical Applications
Soap bubbles and foam form because soaps reduce surface tension. Water droplets on hydrophobic surfaces (lotus leaves, car wax) demonstrate surface tension and non-wetting. Detergents reduce water's surface tension, improving penetration into fabric fibers for cleaning. Tears form spheres due to surface tension minimizing surface area. Meniscus formation in graduated cylinders is due to surface tension and capillarity. Flotation of water striders and water beetles on water surface is enabled by surface tension supporting their weight.
Worked Examples
Example 1. Calculate the capillary rise of water in a glass tube of radius 0.5 mm at 20°C. (Surface tension of water = 0.073 N/m, density = 1000 kg/m³, g = 10 m/s², contact angle = 0°)
Solution: Given: r = 0.5 mm = 0.5 × 10^-3 m, T = 0.073 N/m, ρ = 1000 kg/m³, g = 10 m/s², θ = 0° (cos0° = 1)
Using capillary rise formula: h = 2T cosθ / (ρgr)
h = (2 × 0.073 × 1) / (1000 × 10 × 0.5 × 10^-3)
h = 0.146 / 5
h = 0.0292 m = 29.2 mm
Water rises approximately 29.2 mm (about 3 cm) in a capillary tube of 0.5 mm radius.
Example 2. Explain why mercury does not wet glass but water does, in terms of intermolecular forces.
Solution: Water wets glass:
- Adhesive forces between water and glass > Cohesive forces within water
- Water molecules are attracted more strongly to glass than to each other
- Water spreads on glass surface, forms a meniscus curved upward
- Water rises in glass capillaries
Mercury does not wet glass:
- Cohesive forces within mercury > Adhesive forces between mercury and glass
- Mercury molecules are attracted more strongly to each other than to glass
- Mercury forms droplets, does not spread on glass surface
- Mercury is depressed (sinks) in glass capillaries
Contact angle:
- Water on glass: θ < 90° (wetting liquid)
- Mercury on glass: θ > 90° (non-wetting liquid)
This difference determines whether liquid rises or falls in capillaries.
Example 3. A soap bubble of radius 2 cm is formed from water containing soap. Calculate the minimum pressure difference between inside and outside the bubble. (Surface tension of soap solution ≈ 25 mN/m = 0.025 N/m)
Solution: A soap bubble has two surfaces (inner and outer), each contributing to pressure difference.
For a single liquid surface: ΔP = 2T/r
For a soap bubble with two surfaces: ΔP = 4T/r
Given: T = 0.025 N/m, r = 2 cm = 0.02 m
ΔP = 4 × 0.025 / 0.02
ΔP = 0.1 / 0.02
ΔP = 5 Pa
The pressure inside the bubble is 5 Pa higher than outside. This is why small bubbles collapse (higher pressure difference) and big bubbles float longer.
Where You See This in Real Life
- Traditional Indian vessels made of clay or copper use water's surface tension. Water remains in containers even with small holes because surface tension holds the water in.
- When washing clothes in Indian homes, adding detergent reduces water's surface tension, allowing better penetration into fabric fibers and more effective cleaning.
- The lotus flower (national flower of India) repels water due to its hydrophobic surface, utilizing surface tension to keep water from wetting the leaves.
- Indian children playing with soap bubbles observe surface tension in action as bubbles form and float, demonstrating the elastic nature of the liquid surface.
Common Mistakes Students Make
- Confusing surface tension with surface energy. Surface tension is force per unit length; surface energy is energy per unit area. They are related but different concepts.
- Not considering that soap bubbles have two surfaces. Pressure formula uses 4T/r for bubbles, not 2T/r.
- Assuming all liquids behave like water in capillaries. Mercury behaves differently (depressed) because it does not wet glass.
- Thinking surface tension only causes spherical shapes. It actually minimizes surface area for any shape - droplets, bubbles, and meniscus in tubes.
Frequently Asked Questions
Why is water's surface tension so much higher than most liquids?
Water molecules form strong hydrogen bonds with each other. These hydrogen bonds create very strong intermolecular attractive forces, resulting in high surface tension (~72 mN/m at 20°C). Most other liquids lack such strong intermolecular forces.
How does adding soap reduce surface tension of water?
Soap molecules have both hydrophilic (water-loving) and hydrophobic (oil-loving) parts. They accumulate at the water surface with hydrophobic parts pointing outward, disrupting the regular hydrogen bond network of water molecules. This reduces the net inward force and thus the surface tension.
Why do insects like water striders not sink even though they are denser than water?
Water striders have water-repellent (hydrophobic) legs that do not break the water surface. The weight of the insect creates dimples in the water surface but does not penetrate it. The surface tension of water is strong enough to support the insect's weight distributed over the leg area.