A drop of oil placed on the surface of water spreads out. But a drop of water placed on oil contracts to a sph

II. Short Answer Questions · Q28

Q. A drop of oil placed on the surface of water spreads out. But a drop of water placed on oil contracts to a spherical shape. Why?

Puducherry TnboardTextbookSubjectiveImportance★★★★★

73% · 61/84 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

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.

Important

Surface tension ( $\gamma$ or $T$ ) is defined as the force per unit length acting along the surface, perpendicular to any line drawn on it.

$\gamma = \frac{F}{L}$

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 $F = \gamma L$ . 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:

\text{Work done} = \gamma \times \text{increase in area}

Tip

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. …

Start your 14-day free trial to unlock the full solution →

Step 1. Whether a liquid spreads over another liquid's surface or instead contracts into a bead is decided by the balance of the relevant interfacial surface tensions -- the equation $\cos\theta=(T_{sa}-T_{sl})/T_{la}$ -type balance that fixes the angle of contact.

Step 2. For a drop of OIL placed on WATER, the combination of oil's own surface tension (against air) and the oil-water interfacial tension is LESS than water's surface tension against air; no equilibrium contact angle exists in this case (this is the 'no equilibrium, complete spreading' condition), so the oil spreads out indefinitely to reduce the exposed high-surface-tension water surface, forming a thin film. …

Start your 14-day free trial to unlock the full solution →

←Previous question · 7.8Q27 — What do you mean by capillarity or capillary action?→Next question · 7.8Q29 — State the principle and usage of Venturimeter.

Q.A drop of oil placed on the surface of water spreads out. But a drop of water placed on oil contracts to a spherical shape. Why?

🎓Unlock everything free for 14 days

Unlock everything free for 14 days