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Q9.2(a):
Explain why (a) The angle of contact of mercury with glass is obtuse, while that of water with glass is acute.

Solution :

Theoretical Foundation: The Young-Dupré Equation

The behavior of a liquid meniscus at the interface of a solid surface is governed by the vector balance of three interfacial tensions acting at the line of contact. These tensions are defined as:

  • $\gamma_{sl}$: Interfacial tension between the solid and the liquid.
  • $\gamma_{sg}$: Interfacial tension between the solid and the gas (air).
  • $\gamma_{lg}$: Interfacial tension between the liquid and the gas.

The equilibrium condition for the angle of contact $\theta$ is given by the Young's relation: $\cos \theta = \frac{\gamma_{sg} - \gamma_{sl}}{\gamma_{lg}}$ [Per the principle of static equilibrium of surface tensions].

Step 1: Analyzing Water-Glass Interface (Acute Angle)

In the case of water on glass, the adhesive forces (forces between water molecules and glass molecules) are significantly stronger than the cohesive forces (forces between water molecules themselves).

  • Since adhesive forces are dominant, $\gamma_{sg} > \gamma_{sl}$.
  • Substituting into the relation: $\cos \theta = \frac{(\text{positive value})}{\gamma_{lg}}$.
  • This results in $\cos \theta > 0$, implying that $\theta < 90^\circ$ [Mathematical definition of the first quadrant in trigonometry].

Consequently, the liquid spreads to maximize contact with the solid surface, resulting in a concave meniscus and an acute contact angle.

Step 2: Analyzing Mercury-Glass Interface (Obtuse Angle)

In the case of mercury on glass, the cohesive forces (forces between mercury atoms) are extremely strong compared to the adhesive forces (forces between mercury and glass molecules).

  • The liquid minimizes its surface area to minimize potential energy [Per the principle of minimum energy configuration].
  • In this system, $\gamma_{sl} > \gamma_{sg}$.
  • Substituting into the relation: $\cos \theta = \frac{(\text{negative value})}{\gamma_{lg}}$.
  • This results in $\cos \theta < 0$, implying that $90^\circ < \theta < 180^\circ$ [Mathematical definition of the second quadrant in trigonometry].

Consequently, the mercury tends to bead up to minimize surface contact with the glass, resulting in a convex meniscus and an obtuse contact angle.

Visual Representation of Contact Angles

Water on Glass θ < 90° Mercury on Glass θ > 90°

Comparison Table

Feature Water-Glass Mercury-Glass
Dominant Force Adhesion Cohesion
Contact Angle Acute ($\theta < 90^\circ$) Obtuse ($\theta > 90^\circ$)
Meniscus Shape Concave Convex

Final Solution: The angle of contact is determined by the balance of surface tensions ($\gamma_{sg}, \gamma_{sl}, \gamma_{lg}$). For water, adhesive forces exceed cohesive forces ($\gamma_{sg} > \gamma_{sl}$), yielding $\cos \theta > 0$ (acute). For mercury, cohesive forces dominate ($\gamma_{sl} > \gamma_{sg}$), yielding $\cos \theta < 0$ (obtuse).


More Questions from Class 11 Physics Mechanical Properties of Fluids EXERCISES


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