JAMB 2018 Physics · Question 4

Mercury which is spilled onto a glass surface forms ball-like shapes because____.

Options

A.Mercury is a metal
B.The cohesive force between its molecules is greater than the adhesive force between mercury and glass molecules
C.The cohesive force between its molecules is less than the adhesive force between mercury and glass molecules
D.The viscosity of mercury is height

Answer: B. The cohesive force between its molecules is greater than the adhesive force between mercury and glass molecules

Explanation written with AI and checked against the answer key

Analysis

The question asks for the physical reason why mercury, when spilled on a glass surface, does not spread out but instead forms spherical droplets (ball-like shapes). This is a classic question on Surface Tension and the interaction between intermolecular forces.

Concept Explanation

To understand this, we must distinguish between two types of forces:

Cohesive Force: The force of attraction between molecules of the same substance (e.g., mercury molecules attracting other mercury molecules).
Adhesive Force: The force of attraction between molecules of different substances (e.g., mercury molecules attracting glass molecules).

When Cohesive forces > Adhesive forces: The liquid molecules prefer to stick to each other rather than the surface. This causes the liquid to bead up and minimize its surface area, forming a sphere (the shape with the smallest surface area for a given volume). This is why mercury does not "wet" glass.
When Adhesive forces > Cohesive forces: The liquid molecules are more attracted to the surface than to each other. This causes the liquid to spread out and "wet" the surface (e.g., water on clean glass).

Step-by-Step Solution

Observe the behavior: Mercury forms beads (spheres) on glass. This means it is trying to minimize contact with the glass surface.
Apply the principle: Since the mercury is pulling itself inward to form a sphere, the internal attraction (cohesion) must be significantly stronger than the attraction to the glass (adhesion).
Evaluate the options: • Option A (It is a metal) is a fact, but it doesn't explain the shape. • Option B states that cohesive force is greater than adhesive force. This aligns with our physical principle. • Option C is the opposite of what happens with mercury; it describes what happens with water on glass. • Option D (Viscosity) relates to a fluid's resistance to flow, not its shape-forming properties on a surface.

Conclusion

Exam Tip: Remember the "Water vs. Mercury" rule:

Water on glass: Adhesive > Cohesive (Wets the surface).
Mercury on glass: Cohesive > Adhesive (Does not wet the surface).

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