Water’s high surface tension is one of its most famous and important physical properties. In IB Chemistry, understanding why this happens requires connecting intermolecular forces, molecular structure, polarity, and hydrogen bonding. Examiners often test this concept through data interpretation, explanation questions, and real-world applications. Mastering the reasoning behind surface tension will strengthen your understanding of both physical chemistry and molecular interactions across the course.
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Quick Start Checklist
Before diving deeper, make sure you understand:
- Water molecules are highly polar.
- Strong hydrogen bonds form between molecules.
- Hydrogen bonding pulls water molecules tightly together.
- This creates a “skin-like” surface called surface tension.
- Water’s surface tension is higher than most other liquids.
These ideas form the backbone of Paper 2 explanation questions.
What Is Surface Tension?
Surface tension is the force that makes the surface of a liquid behave like a stretched elastic membrane. It occurs because molecules at the surface experience unequal forces:
- Molecules inside the liquid experience attractions in all directions.
- Molecules at the surface experience a net inward attraction.
The stronger the intermolecular forces, the stronger the pull toward the interior—resulting in higher surface tension.
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Water’s Polarity
Water’s molecular polarity is the first piece of the puzzle. The bent shape of H₂O and the electronegativity difference between oxygen and hydrogen give the molecule a strong dipole moment:
- Oxygen end: partial negative
- Hydrogen end: partial positive
This creates intense attractions between molecules, setting the stage for hydrogen bonding.
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Hydrogen Bonding: The Key Reason
Water’s high surface tension is primarily caused by hydrogen bonding, the strongest type of intermolecular force (excluding ionic and metallic bonding). Hydrogen bonds form when:
- A hydrogen atom bonded to O, N, or F
- Is attracted to a lone pair on another electronegative atom nearby
In water, every molecule can form up to four hydrogen bonds:
- Two as a hydrogen donor
- Two as an oxygen acceptor
This extensive network of hydrogen bonds creates:
- Strong cohesion
- Tight packing at the surface
- High surface tension
Few liquids form such extensive hydrogen bonding, which is why water has unusually high surface tension.
Effects of High Surface Tension
Water’s high surface tension leads to many observable phenomena:
- Water droplets form nearly spherical shapes.
- Small insects (like water striders) can walk on its surface.
- Capillary action helps water move through plant vessels.
- Water resists external force more than most liquids.
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Comparison With Other Liquids
Most liquids—such as ethanol, acetone, or oils—exhibit much lower surface tension because they lack strong hydrogen bonding. Their intermolecular forces are weaker (often only London dispersion forces), so their molecules are not pulled inward as strongly.
This comparison often appears in data-based questions, so being able to justify differences using intermolecular forces is essential.
Frequently Asked Questions
Why is water’s surface tension stronger at lower temperatures?
At lower temperatures, molecules move more slowly, meaning hydrogen bonds are harder to break. This strengthens cohesion and increases surface tension.
Does salt affect water’s surface tension?
Yes. Adding ions often increases surface tension by strengthening attractions between molecules at the surface. This is relevant in many IA experiments involving solutions.
Can detergents reduce water’s surface tension?
Absolutely. Detergents contain surfactants that disrupt hydrogen bonding, reducing surface tension and allowing water to spread more easily—important in cleaning and industrial processes.
Conclusion
Water’s high surface tension is caused by strong hydrogen bonding between highly polar water molecules. This creates powerful cohesive forces, pulling surface molecules inward and making the surface behave like a stretched membrane. Understanding this concept strengthens your mastery of intermolecular forces, molecular polarity, and physical chemistry—core skills in IB Chemistry.
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