On a clean glass plate, a drop of water spreads into a thin layer but a drop of mercury remains almost spherical. Which explanation best accounts for this difference in behaviour?

Introduction / Context:
This question explores surface tension and intermolecular forces, specifically cohesion and adhesion. Liquids on solid surfaces exhibit characteristic shapes depending on the balance between cohesive forces within the liquid and adhesive forces between the liquid and the solid. Understanding why water tends to wet glass and spread out, while mercury forms nearly spherical droplets, illustrates these concepts and is a classic example in physics and chemistry.

Given Data / Assumptions:

• A drop of water on clean glass spreads to form a thin layer.
• A drop of mercury on the same glass remains almost spherical.
• Options mention cohesion, adhesion, density and metallic nature.
• Cohesion refers to attractive forces between molecules of the same substance.
• Adhesion refers to attractive forces between molecules of different substances.

Concept / Approach:
When adhesion between a liquid and a solid surface is stronger than cohesion within the liquid, the liquid tends to spread and wet the surface. When cohesion within the liquid dominates over adhesion to the surface, the liquid tends to bead up and form droplets with minimal contact area, often appearing spherical due to surface tension. Water has strong adhesive interactions with glass because both involve polar molecules and hydrogen bonding, so water spreads. Mercury has strong cohesive metallic bonding within the liquid, while its adhesion to glass is relatively weak, so droplets minimise contact with the glass and stay nearly spherical.

Step-by-Step Solution:
Step 1: Consider water on glass. Water molecules are polar and can form hydrogen bonds and strong attractions to the polar surface of glass (silica). Step 2: Adhesive forces between water and glass exceed cohesive forces between water molecules at the surface, so water spreads out to maximise contact with glass. Step 3: Now consider mercury on glass. Mercury atoms are held together by strong metallic bonding, producing very strong cohesive forces within the liquid metal. Step 4: Adhesion between mercury and glass is relatively weak compared with the cohesion within mercury. Step 5: As a result, the mercury droplet tries to minimise its surface area and contact with the glass, forming an almost spherical shape. Step 6: Therefore, the correct explanation is that cohesion of mercury is greater than its adhesion with glass.

Verification / Alternative check:
Capillary action experiments support this reasoning. Water in a narrow glass tube rises above the external water level, forming a concave meniscus because adhesion to glass pulls water up along the walls. Mercury in a glass capillary shows a convex meniscus and actually falls below the external mercury level, indicating that cohesion in mercury dominates and the liquid pulls away from the glass walls. These behaviours are textbook evidence that mercury has stronger cohesion than adhesion with glass, while water shows the opposite trend, explaining the shapes of drops on a glass plate.

Why Other Options Are Wrong:
Saying mercury is a metal and therefore must be spherical is not a correct explanation; metallic nature alone does not guarantee spherical shape on all surfaces. Density differences affect gravitational forces but do not directly determine whether a drop spreads or beads up on a surface; surface tension and adhesion are more important. The statement that cohesion of water is greater than its adhesion with glass is opposite to reality; if that were true, water would form droplets rather than spreading. The idea that water has much higher surface tension than mercury is also incorrect; mercury actually has a higher surface tension, which is one reason its drops are so spherical.

Common Pitfalls:
Students sometimes focus on density or metal versus nonmetal classification instead of thinking about intermolecular forces. Another common error is to assume that higher surface tension alone explains everything, ignoring the balance between cohesion and adhesion. To avoid these mistakes, always analyse whether the liquid wets the surface or not. If it wets, adhesion dominates; if it beads up, cohesion dominates. Remember that water wets clean glass (strong adhesion), while mercury does not (strong cohesion), which is exactly what this question illustrates.

Final Answer:
A drop of mercury remains almost spherical on glass because the cohesion of mercury is greater than its adhesion with glass, whereas water has stronger adhesion to glass and spreads.