Which one of the following statements is correct ? .

1. Intermolecular Forces: Cohesion vs. Adhesion (basic)

To understand mechanics at a fundamental level, we must first look at how the tiny particles that make up matter interact with one another. All matter is held together by interparticle attractions. These forces are the 'glue' of the universe, and their strength determines whether a substance behaves as a solid, liquid, or gas Science, Class VIII NCERT, Particulate Nature of Matter, p.101. When we focus on liquids and their interactions with surfaces, we categorize these attractive forces into two types: Cohesion and Adhesion.

Cohesion is the attractive force between molecules of the same substance. It is the internal 'team spirit' that keeps a droplet of water together in a spherical shape. On the other hand, Adhesion is the attractive force between molecules of different substances, such as water molecules and the glass walls of a tube. These are both examples of contact forces, as they emerge from the physical proximity of particles Science, Class VIII NCERT, Exploring Forces, p.66.

The 'tug-of-war' between these two forces determines how a liquid behaves when it touches a solid. If the adhesive force is stronger than the cohesive force, the liquid will 'climb' the surface, wetting it and forming a concave meniscus (a U-shaped curve). If the cohesive force is stronger, the liquid will huddle together, avoiding the surface and forming a convex meniscus (like a dome).

Force Type	Interaction	Example Result
Cohesion	Between similar molecules	Water forming a droplet
Adhesion	Between different molecules	Water sticking to a glass window

Sources: Science, Class VIII NCERT, Particulate Nature of Matter, p.101; Science, Class VIII NCERT, Exploring Forces, p.66

2. Surface Tension: Why Drops are Spherical (basic)

To understand why liquid drops are spherical, we must first look at the interparticle forces of attraction acting within the liquid Science, Class VIII NCERT, Particulate Nature of Matter, p.105. Deep inside a liquid, a molecule is pulled equally in all directions by its neighbors. However, a molecule at the surface has no liquid molecules above it. This creates a net inward pull, making the surface behave like a stretched elastic membrane. This property is known as Surface Tension. In nature, systems always seek the state of minimum energy, which for a liquid means minimizing its surface area. Geometrically, for a fixed volume, a sphere has the smallest surface area, which is why free-falling raindrops naturally form spheres. When a drop touches a solid surface, a 'tug-of-war' begins between two types of forces: Cohesive forces (attraction between similar molecules, like water-to-water) and Adhesive forces (attraction between different molecules, like water-to-glass). The outcome of this struggle is measured by the Angle of Contact—the angle formed between the liquid's surface and the solid surface, measured inside the liquid. This angle determines whether a liquid will 'wet' the surface or bead up into a sphere.

Feature	Water on Glass	Mercury on Glass
Dominant Force	Adhesion (Water-Glass) > Cohesion	Cohesion (Mercury-Mercury) > Adhesion
Meniscus Shape	Concave (curvatures upward)	Convex (curvatures downward)
Angle of Contact	Acute (typically 8° to 18°)	Obtuse (approximately 138°)
Result	Wets the glass; spreads out.	Does not wet; beads up into spheres.

Sources: Science, Class VIII NCERT (Revised ed 2025), Particulate Nature of Matter, p.105

3. Factors Affecting Surface Properties (intermediate)

When we look at how a liquid interacts with a solid surface—like water in a glass or rain on a leaf—we are observing a tug-of-war between two fundamental molecular forces: Cohesion and Adhesion. Cohesion is the force of attraction between molecules of the same substance (e.g., water molecule to water molecule), while Adhesion is the attraction between molecules of different substances (e.g., water molecule to glass). The balance between these forces determines the surface properties of the liquid, most notably the angle of contact.

The angle of contact is defined as the angle between the tangent to the liquid surface and the solid surface at the point of contact, measured inside the liquid. This angle tells us how well a liquid "wets" a surface. For instance, in a Six’s thermometer, alcohol is used because its surface tension allows it to drag an indicator along the tube as it contracts Certificate Physical and Human Geography, Weather, p.120. This interaction is highly dependent on whether the liquid prefers to stick to itself or the container.

Feature	Water on Glass	Mercury on Glass
Dominant Force	Adhesion > Cohesion	Cohesion > Adhesion
Wetting Ability	Wets the surface	Does not wet the surface
Meniscus Shape	Concave (curvatures upward)	Convex (curvatures downward)
Angle of Contact	Acute (typically 8° to 18°)	Obtuse (approximately 138°)

External factors also play a critical role. Temperature is a major influencer; as temperature rises, the kinetic energy of particles increases, causing them to move more vigorously and decreasing the interparticle forces of attraction Science Class VIII, Particulate Nature of Matter, p.105. This typically reduces surface tension. Furthermore, environmental factors like acid rain can chemically alter surface properties, leading to surface erosion and the formation of crusts on materials like glass and ceramics Environment Shankar IAS, Environmental Pollution, p.105.

Sources: Certificate Physical and Human Geography, Weather, p.120; Science Class VIII NCERT, Particulate Nature of Matter, p.105; Environment Shankar IAS Academy, Environmental Pollution, p.105

4. Viscosity: Internal Friction in Fluids (intermediate)

When we talk about viscosity, we are essentially discussing the "thickness" or the internal friction of a fluid. Imagine pouring water versus pouring sugar syrup (Chashni). While both are liquids and have a definite volume, the sugar syrup moves much more slowly. This is because viscosity represents the resistance a fluid offers to its own flow. In a flowing fluid, different layers move at different velocities, and viscosity is the force that tries to prevent these layers from sliding past each other.

To understand why this happens, we must look at the particles. We know that liquids do not have a fixed shape because their particles are free to move (Science, Class VIII, NCERT, Particulate Nature of Matter, p.104). However, these particles still exert intermolecular forces on one another. In highly viscous fluids like Chashni, the presence of a large amount of solute (sugar) dissolved in the solvent (water) increases these internal attractions and collisions (Science, Class VIII, NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.136), making it harder for the layers to slide, thus increasing friction.

Temperature plays a crucial role in how fluids flow. Generally, as the temperature increases, the kinetic energy of the particles increases, allowing them to overcome their internal attractive forces more easily. This causes the viscosity of a liquid to decrease, making it "thinner" and easier to pour. This is a common observation when heating cold oil or honey to make them flow more readily.

Feature	Low Viscosity (e.g., Water)	High Viscosity (e.g., Honey/Syrup)
Flow Rate	Flows quickly and easily.	Flows slowly and sluggishly.
Internal Friction	Low friction between layers.	High friction between layers.
Molecular Interaction	Weaker resistance to sliding.	Stronger resistance to sliding.

Sources: Science, Class VIII, NCERT, Particulate Nature of Matter, p.104; Science, Class VIII, NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.136

5. Capillarity: Rise and Fall of Liquids (intermediate)

Capillarity is the fascinating ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity. Imagine a very thin glass tube (a capillary) dipped into a bowl of water; you will notice the water level inside the tube rises above the outside level. This phenomenon is a direct result of the interplay between cohesion (the attraction between similar molecules) and adhesion (the attraction between different molecules, like liquid and glass).

Whether a liquid rises or falls depends on the angle of contact. This is the angle formed between the tangent to the liquid surface and the solid surface at the point of contact, measured inside the liquid. When the adhesive force is stronger than the cohesive force, the liquid "wets" the surface, forms a concave meniscus, and the angle of contact is acute (less than 90°). Conversely, if cohesive forces dominate, the liquid avoids the surface, forms a convex meniscus, and the angle of contact is obtuse (greater than 90°).

Feature	Water in Glass	Mercury in Glass
Dominant Force	Adhesion (Liquid-Solid)	Cohesion (Liquid-Liquid)
Angle of Contact	Acute (typically 8°–18°)	Obtuse (approx. 138°)
Action	Capillary Rise	Capillary Fall/Depression
Meniscus Shape	Concave (curved inward)	Convex (curved outward)

In the natural world, capillarity is essential for life. It is the primary mechanism that allows water and dissolved minerals to move upwards through the xylem—thin, tube-like structures in plants—to reach the leaves and flowers Science-Class VII . NCERT(Revised ed 2025), Life Processes in Plants, p.148. Similarly, in our own bodies, the smallest blood vessels are called capillaries, having walls just one-cell thick to facilitate the exchange of materials between blood and tissues Science , class X (NCERT 2025 ed.), Life Processes, p.93. The height to which a liquid rises is inversely proportional to the radius of the tube; the thinner the tube, the higher the rise!

Sources: Science-Class VII . NCERT(Revised ed 2025), Life Processes in Plants, p.148; Science , class X (NCERT 2025 ed.), Life Processes, p.93

6. Angle of Contact and Meniscus Formation (exam-level)

When you pour a liquid into a glass container, you’ll notice that the surface isn’t perfectly flat where it meets the walls; instead, it curves. This curved upper surface of a liquid is called a meniscus Science Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.144. Whether this curve bends upward or downward depends on a microscopic "tug-of-war" between two types of forces: Cohesive forces (attraction between like molecules, such as water-to-water) and Adhesive forces (attraction between unlike molecules, such as water-to-glass).

The Angle of Contact (θ) is the formal way we measure this interaction. It is defined as the angle between the tangent to the liquid surface and the solid surface at the point of contact, always measured inside the liquid. Understanding this angle allows us to predict how a liquid will behave on a surface:

Scenario	Force Comparison	Meniscus Shape	Angle of Contact
Water and Glass	Adhesive > Cohesive	Concave (curved inward)	Acute (θ < 90°)
Mercury and Glass	Cohesive > Adhesive	Convex (curved outward)	Obtuse (θ > 90°)

In the case of water and glass, the water molecules are more attracted to the glass than to each other, causing the liquid to "climb" the walls, creating a concave meniscus. This liquid is said to "wet" the surface. Conversely, mercury atoms are much more attracted to each other than to the glass. They pull away from the walls to form a convex meniscus, resulting in an obtuse angle (typically around 138°). When taking measurements in a lab, it is crucial to read the bottom of a concave meniscus and the top of a convex one to ensure accuracy Science Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.144.

Sources: Science Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.144

7. Solving the Original PYQ (exam-level)

This question is a classic application of the intermolecular forces you’ve just studied. To solve it, you must synthesize two key building blocks: cohesive forces (attraction between similar molecules) and adhesive forces (attraction between different molecules). According to the principles detailed in Fluids-surface_tension_and_viscosity-GET4-AD.pdf, the angle of contact is essentially a physical manifestation of the "tug-of-war" between these forces at the solid-liquid interface. When adhesion wins, the liquid "wants" to spread; when cohesion wins, it "wants" to pull away.

Let's walk through the reasoning: In the case of water and glass, the adhesive force is stronger than the cohesive force, causing the water to "wet" the surface and form a concave meniscus. Since the angle is measured inside the liquid, this curvature results in an acute angle. Conversely, for mercury and glass, the cohesive forces are dominant. Mercury molecules would rather stick to each other than to the glass, forming a convex meniscus and pulling away into an obtuse angle. This logical chain directly confirms that Option (A) is the correct answer.

UPSC often uses symmetry traps or reversals in options like (B) and (C) to test your conceptual clarity. Option (B) is a simple reversal designed to catch students who might have confused which liquid "wets" the glass. Option (C) ignores the fundamental chemical differences between substances, assuming a uniform behavior that does not exist in nature. As a civil services aspirant, always remember that the nature of the substance determines the interaction; wetting liquids always lead to acute angles, while non-wetting liquids lead to obtuse ones.