Q.Why can a water spider walk comfortably on the surface of still water?
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Surface Tension
Imagine a water strider skating across a pond. Its feet don't break the surface — they dent it, as if standing on an invisible, stretchy skin. Or think of a carefully placed paperclip floating on water, even though steel is much denser. That "skin" is real, and the property behind it is surface tension.
At the molecular level, a liquid molecule is pulled equally in all directions by its neighbours — except at the surface. A molecule deep inside feels cohesive forces from every side, so the net force is zero. But a molecule at the surface has neighbours only below and to the sides; none above. The result is a net inward pull. The surface is therefore in a state of tension — it constantly tries to shrink to the smallest possible area, just like a stretched rubber sheet.
Surface tension (γ or T) is defined as the force per unit length acting along the surface, perpendicular to any line drawn on it.
γ=LF
Its SI unit is N/m (newton per metre).
This definition is precise: if you imagine cutting an imaginary line of length L on the liquid surface, the surface on one side pulls the other side with a force F=γL. That's why a soap film in a wire frame pulls inward — the film's surface tension acts along the edges.
A more intuitive way to see it: surface tension gives the liquid surface an energy cost for increasing its area. To create more surface, you must bring molecules from the interior to the surface, working against the inward pull. The work done per unit area is numerically equal to the surface tension:
Work done=γ×increase in area
For quick problems, remember: surface tension is both a force-per-length and an energy-per-area. They are the same number in different units (J/m² = N/m).
This explains everyday phenomena:
- Drops are spherical — a sphere has the smallest surface area for a given volume, so surface tension pulls the liquid into that shape. …
The same surface film that holds a greased blade: …
Non-wetting legs + surface tension = walking on the film.
A water spider (pond skater) is extremely light and stands on several spread legs tipped with waxy, non-wetting hairs. Each leg presses a small dimple into the surface; the stretched film's tension acts along the dimple rim with an upward component, and summed over all legs this upward force exceeds the spider's weight. The legs never break through, so the spider strides across the 'membrane' — the standard living illustration of surface tension (§2.4). …
Scale analysis: weight grows with volume (L³) but surface-tension support with contact length (L) — which is why only centimetre-or-sma …
Thinking the spider floats by buoyancy or swims — its body never touches the water; remove …
Showing the 12 most recent of 33 on this concept.
- CBSE 2026Set ANNUAL1 markMCQQ.Insects are able to run on the surface of water because(a) surface tension makes its surface behave as an elastic membrane(b) insects swim on water(c) insects have less weight(d) of Archimedes' upthrust
›Reveal solutionSolution
Surface tension causes the liquid surface to behave like a taut elastic membrane; the upward tension forces along the depressed surface around the insect's legs are enough to balance its (very small) weight, so it does not sink.
Surface tension arises because molecules at a liquid's surface experience a net inward pull from neighbouring molecules (since they lack neighbours above them), causing the surface to minimise its area and behave like a stretched elastic skin.
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- CBSE 2026Set ANNUAL1 markMCQQ.The contact angle between pure water and clean glass is(a) 0°(b) 8°(c) 45°(d) 90°
›Reveal solutionSolution
Pure water on clean glass has a contact angle of about 0 degrees. Answer (A).
The angle of contact depends on the liquid-solid pair. Pure water strongly wets a clean glass surface (adhesive forces dominate), so the liquid spreads and the contact angle is essentially 0 …
- CBSE 2025Set ANNUAL1 markQ.At what temperature the surface tension of a liquid becomes zero?
›Reveal solutionSolution
Surface tension decreases as temperature rises and vanishes at the critical temperature.
Surface tension arises from the net inward cohesive force on surface molecules, which decreases as temperature increases (molecules move apart and cohesive forces weaken). As the temperature of a liquid approaches its critical temperature Tc, the distinction between the liquid and its v …
- CBSE 2025Set ANNUAL1 markMCQQ.SI unit of surface tension is (A) newton-m^-2 (B) newton-m^-1 (C) kg-m^-1 (D) kg-m^-2
›Reveal solutionSolution
Surface tension is force per unit length, so its SI unit is N/m.
Surface tension T is defined as the force F acting perpendicular to, and along, a unit length of an imaginary line drawn on the liquid surface:
T=lF
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- CBSE 2025Set ANNUAL1 markMCQQ.The cause of surface tension is (A) Friction force (B) Cohesive force (C) Adhesive force (D) Nature of liquid
›Reveal solutionSolution
Surface tension is caused by cohesive forces between the liquid's own molecules.
Inside a liquid, a molecule is surrounded on all sides by other liquid molecules and experiences balanced cohesive forces in every direction, giving zero net force. A molecule at the surface, however, has liquid molecules only below and to the sides (not above), so it experiences a net inward (cohesive) pull. This net inward pull on surface molecules causes the surface to behave like a stretched elastic membrane, which is surface …
- CBSE 2025Set ANNUAL1 markMCQQ.With increase of temperature the surface tension of a liquid (A) increases (B) decreases (C) remains same (D) increases rapidly
›Reveal solutionSolution
Surface tension of a liquid decreases with rising temperature.
Surface tension arises from cohesive intermolecular forces. As temperature increases, the molecules gain more kinetic energy, move faster, and the average intermolecular separation increases, which weakens the cohesive attraction between them. This directly reduces the surface …
- CBSE 2025Set ANNUAL1 markMCQQ.On increasing the temperature of water, its surface tension(a) Increases(b) Decreases(c) Remains constant(d) Shows irregular behaviour
›Reveal solutionSolution
Surface tension of water decreases with increasing temperature.
Surface tension is caused by the net inward pull on surface molecules from intermolecular attractions (in water, largely hydrogen bonding). As temperature rises, molecules gain kinetic energy and move faster, weakening the effect of these intermolecular attractive forces. This reduces the net in …
- CBSE 2025Set ANNUAL1 markMCQQ.Mercury does not wet glass. This property of liquid is known as(a) adhesion(b) surface tension(c) viscosity(d) compressibility
›Reveal solutionSolution
Whether a liquid wets a solid surface is governed by the relative strength of cohesive forces (within the liquid) versus adhesive forces (between liquid and solid); mercury's weak adhesion to glass (compared to its strong internal cohesion) is why it beads up instead of spreading (non-wetting), which is directly reflected in its obtuse angle of contact with glass.
A liquid 'wets' a solid surface when the adhesive force between the liquid and solid molecules is strong enough to overcome the liquid's own cohesive forces, causing it to spread out (e.g. water on clean glass).
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- CBSE 2025Set ANNUAL1 markMCQQ.Assertion (A) : A small droplet of liquid is spherical in shape. Reason (R) : Surface tension of the liquid tries to minimize the surface area of the droplet.(a) Both (A) and (R) are true, and (R) is correct explanation of (A).(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).(c) (A) is true, but (R) is false(d) (A) is false, but (R) is true
›Reveal solutionSolution
Both statements are true, and the surface-tension explanation correctly accounts for why droplets are spherical.
Assertion: A small droplet of liquid is spherical — this is true; we observe this with dew drops, mercury droplets, etc.
Reason: Surface tension tries to minimize the surface area of the droplet — this is also true. Surface tension arises because molecules at the surface of a liquid experience a net inward force (they have fewer neighbouring molecules than those in the bulk), so the surface behaves like a stretched elastic membrane that tends to contract to the smallest possible area for the enclosed volume.
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- CBSE 2025Set hz1 markMCQQ.A needle floats on the surface of water because of:(a) Lighter weight(b) Surface tension(c) Adhesive force(d) Viscosity
›Reveal solutionSolution
A needle placed gently on water floats because the surface tension of water acts like a stretched elastic membrane, supporting the needle's weight even though the needle's material is denser than water.
Water molecules at the free surface experience a net inward pull from neighbouring molecules (since they have no water above them), which makes the surface behave like a stretched elastic skin -- this property is called surface tension.
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- CBSE 2025Set hz1 markMCQQ.The meniscus of mercury in a glass tube is:(a) Concave upward(b) Convex upward(c) Plane(d) None of the above
›Reveal solutionSolution
Mercury does not wet glass, so cohesion (mercury-mercury attraction) dominates over adhesion (mercury-glass attraction), producing a convex meniscus that curves upward in the middle of the tube.
Whether a liquid's meniscus is concave or convex depends on the relative strength of cohesive forces (between the liquid's own molecules) and adhesive forces (between the liquid and the container wall):
- If adhesive force > cohesive force (e.g., water in glass), the liquid wets the glass, climbs up along the walls, and the meniscus is concave (curves downward in the middle, i.e., lower at the centre than at the edges). …
- CBSE 2025Set ANNUAL1 markQ.A glass rod coated with wax does not become wet when dipped in water. Why?
›Reveal solutionSolution
Whether a liquid wets a solid surface depends on the relative strength of the liquid-liquid cohesive force and the solid-liquid adhesive force, expressed through the angle of contact θ.
A surface is wetted by a liquid when the adhesive force between the liquid and the solid exceeds the cohesive force within the liquid, giving an acute angle of contact (θ<90°), as with clean glass and water.
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