Q.Define the following:

Showing the 12 most recent of 35 on this concept.

1. CBSE 2026Set ANNUAL1 markMCQ
   Q.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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2. CBSE 2026Set ANNUAL1 markMCQ
   Q.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 …

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3. CBSE 2025Set ANNUAL1 mark
   Q.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 $T_c$, the distinction between the liquid and its v …

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4. CBSE 2025Set ANNUAL1 markMCQ
   Q.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:

   $T = \dfrac{F}{l}$

   …

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5. CBSE 2025Set ANNUAL1 markMCQ
   Q.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 …

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6. CBSE 2025Set ANNUAL1 markMCQ
   Q.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 …

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7. CBSE 2025Set ANNUAL1 markMCQ
   Q.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 …

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8. CBSE 2025Set ANNUAL1 markMCQ
   Q.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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9. CBSE 2025Set ANNUAL1 markMCQ
   Q.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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10. CBSE 2025Set hz1 markMCQ
    Q.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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11. CBSE 2025Set hz1 markMCQ
    Q.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). …

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12. CBSE 2025Set ANNUAL1 mark
    Q.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 $\theta$.

    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 ($\theta < 90°$), as with clean glass and water.

    …

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