Q.Why does a greased razor blade float on the surface of water?
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.
- Capillary rise — in a narrow tube, the curved meniscus (due to surface tension) creates a pressure difference that lifts the liquid.
- Detergents — they lower surface tension, allowing water to spread and wet surfaces instead of beading up.
A common mistake: thinking surface tension is a force on the surface. It is a force within the surface, tangential to it. The net effect on the liquid as a whole is to minimise area, not to push things down.
So when you see a water droplet clinging to a leaf or a needle floating on water, you're watching the cohesive pull of molecules — a microscopic tug-of-war that makes the surface behave like a stretched membrane.
Surface tension is a headline topic in the NCERT/CBSE Class 11 Physics chapter on mechanical properties of fluids, and "surface tension: definition, formula & real-world examples" ranks among the most searched physics concepts at this level. It is also a frequently tested topic in JEE Main and NEET, often paired with surface energy and capillary rise numericals.
A blade denser than water can still rest ON water:
Because the water surface behaves like a stretched elastic membrane: the greased (non-wetting) blade depresses the surface, and the surface tension's upward components support its weight — the blade floats on the SURFACE FILM, not by buoyancy.
The surface film's tension, not buoyancy, holds the blade up.
Steel is nearly eight times denser than water, so the blade cannot float by buoyancy. Laid gently flat, however, it rests in a small depression of the surface: the grease prevents wetting, the surface curves under the blade like a loaded trampoline, and the surface-tension forces along the rim of the depression have upward components that balance the blade's weight (§2.4). Pierce the film — or wet the blade — and it sinks at once.
Because the water surface behaves like a stretched elastic membrane: the greased (non-wetting) blade depresses the surface, and the surface tension's upward components support its weight — the blade floats on the SURFACE FILM, not by buoyancy.
Watch the meniscus: around a floating greased blade the surface visibly dips down; around a wetted (sinking) blade it climbs up — the sign of the contact angle decides support vs capture.
Calling this buoyancy/Archimedes — the displaced water is far too little; the support is the film's tension.
Showing the 12 most recent of 35 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.
When a small, light insect (like a water strider) stands on water, its legs create small depressions in the surface. The surface tension along the curved, depressed surface produces an upward restoring force (much like stretching an elastic membrane creates a restoring tension) that is sufficient to support the insect's small weight, so it doesn't break through the surface. This is NOT swimming, nor is it primarily about the insect's weight being negligible on its own (many small objects heavier than these insects still sink), nor is it Archimedes' upthrust (which relies on buoyancy from submerged volume, not surface effects) -- the defining phenomenon is surface tension acting like an elastic membrane.
✓Final answer(a) surface tension makes its surface behave as an elastic membrane.
- 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 degrees.
This small contact angle is why water rises well in clean glass capillaries.
✓Final answer(A) 0 degrees.
- 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 vapour disappears (the liquid–vapour surface itself vanishes), so the surface tension of the liquid falls to zero at T=Tc.
✓Final answerAt the critical temperature.
- 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
Since F is in newton and l is in metre, the SI unit of T is newton per metre (N m⁻¹), which is dimensionally equivalent to J/m² (energy per unit area).
✓Final answer(B) newton-m⁻¹.
- 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 tension.
(Adhesive force is the attraction between different substances — e.g. liquid and container wall — and explains phenomena like wetting/capillary rise, not surface tension itself.)
✓Final answer(B) Cohesive force.
- 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 tension. At the critical temperature of the liquid, cohesive forces effectively vanish and surface tension becomes zero.
✓Final answer(B) decreases.
- 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 inward pull at the surface, so surface tension decreases with increasing temperature (and becomes zero at the critical temperature).
✓Final answer(B) Decreases.
- 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).
Mercury, by contrast, has very strong internal cohesive forces (metallic bonding between mercury atoms) but comparatively weak adhesive attraction to glass. Because cohesion dominates over adhesion in this liquid-solid pair, mercury pulls itself into beads rather than spreading across the glass -- it does not wet the surface.
Of the given options, this non-wetting behaviour is fundamentally a statement about the (in this case, weak) adhesive property between the liquid and the solid surface.
✓Final answer(a) adhesion.
- 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.
Of all 3-dimensional shapes with a given volume, a sphere has the minimum surface area. Since surface tension drives the droplet toward minimum surface area (for a fixed volume set by the amount of liquid), the droplet naturally assumes a spherical shape. So the Reason correctly and directly explains the Assertion.
✓Final answerThe correct option is (a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
- 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.
When a needle is placed carefully (not dropped) on the water surface, it slightly depresses the surface without breaking it. The surface tension force acts tangentially along the line of contact around the needle, and its vertical component balances the weight of the needle. Since this vertical component of the surface-tension force can support only a limited weight, if the needle is too heavy (or the surface film is broken), it sinks.
This is not due to lighter weight (steel is denser than water), adhesive force (which would pull the needle down into the water, not hold it up), or viscosity (which resists relative motion within the fluid, not surface floating).
✓Final answerThe correct option is (b) Surface tension.
- 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).
- If cohesive force > adhesive force (e.g., mercury in glass), the liquid does not wet the glass, is pulled away from the walls, and the meniscus is convex (bulges upward in the middle, i.e., higher at the centre than at the edges).
Mercury has very strong cohesive forces between its own atoms (a metallic liquid) compared to its adhesion to glass, so it forms a convex-upward meniscus, and this is also why mercury shows capillary depression rather than capillary rise in a narrow glass tube.
✓Final answerThe correct option is (b) Convex upward.
- 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.
Wax is a hydrophobic substance: the adhesive force between the wax surface and water molecules is much weaker than the cohesive force between the water molecules themselves. As a result, the angle of contact between water and a waxed surface is obtuse (θ>90°). Water then tends to pull itself into droplets and roll off, rather than spreading over and sticking to the surface — the wax-coated rod does not become wet.
✓Final answerBecause wax-water adhesion is weaker than water's own cohesion, the angle of contact is obtuse (>90°), so water beads up and does not wet the wax-coated rod
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