Which one among the following does not wet the walls of the glass vessel in which it is kept ?

1. Intermolecular Forces: Cohesion and Adhesion (basic)

At the heart of mechanics lies a simple truth: matter is not just a collection of loose particles, but is held together by invisible 'glues' called interparticle attractions. These forces are attractive in nature and their strength determines whether a substance remains a rigid solid or flows as a liquid Science, Class VIII, Particulate Nature of Matter, p.101. In solids, these interactions are incredibly strong, keeping particles closely packed and giving the object a fixed shape Science, Class VIII, Particulate Nature of Matter, p.113. When we zoom in on how these forces behave at the boundary between two different materials, we encounter the two siblings of intermolecular forces: Cohesion and Adhesion.

Cohesion is the force of attraction between molecules of the same substance. It is the reason water forms droplets rather than just becoming a thin gas. Adhesion, on the other hand, is the attraction between molecules of different substances—like when you use an adhesive material to fix a sheet of paper to a board Science, Class X, Magnetic Effects of Electric Current, p.196. These forces often require physical contact to act effectively, making them a fundamental type of contact force Science, Class VIII, Exploring Forces, p.66.

Whether a liquid 'wets' a surface or beads up depends on the competitive balance between these two forces. If the adhesive forces between the liquid and the surface are stronger than the liquid's internal cohesive forces, the liquid will spread out and 'wet' the surface. This is why water wets glass. Conversely, if the cohesive forces are much stronger, the liquid will pull inward and avoid the surface, as seen with mercury, which forms distinct spheres and refuses to wet glass.

Force Type	Acting Between...	Real-world Result
Cohesion	Molecules of the same substance	Surface tension; water forming droplets.
Adhesion	Molecules of different substances	Glue sticking to wood; water wetting a glass.

Sources: Science, Class VIII (NCERT), Particulate Nature of Matter, p.101, 113; Science, Class X (NCERT), Magnetic Effects of Electric Current, p.196; Science, Class VIII (NCERT), Exploring Forces, p.66

2. Surface Tension: The 'Skin' of Liquids (basic)

Imagine a tiny molecule of water deep inside a glass. It is surrounded by other water molecules, all pulling on it equally from every direction. However, a molecule at the very surface has no water molecules above it—only the air. This creates an imbalance: the surface molecules are pulled inward and sideways, causing the surface to contract and act like a stretched elastic membrane. This phenomenon is what we call Surface Tension.

To understand how liquids interact with surfaces, we must look at the tug-of-war between two internal forces:

• Cohesion: The attractive force between molecules of the same substance (e.g., water-water).
• Adhesion: The attractive force between molecules of different substances (e.g., water-glass).

When you pour a liquid into a container, you might notice the surface isn't perfectly flat. As noted in Science, Class VIII NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.144, this curved surface is called a meniscus. If the adhesive forces between the liquid and the glass are stronger than the internal cohesive forces, the liquid 'climbs' the walls, creating a concave meniscus (curved downwards). This is why water wets glass. Conversely, if cohesive forces dominate, the liquid pulls away from the walls to form a convex meniscus (curved upwards), like mercury. Because mercury's internal attraction is so powerful, it refuses to 'wet' the glass and instead forms distinct, rounded beads.

Condition	Dominant Force	Result	Meniscus Shape
Wetting (e.g., Water on Glass)	Adhesion > Cohesion	Liquid spreads out	Concave (U-shaped)
Non-Wetting (e.g., Mercury on Glass)	Cohesion > Adhesion	Liquid beads up	Convex (n-shaped)

In nature, surface tension is the reason why raindrops are spherical. Since a sphere has the smallest surface area for a given volume, surface tension pulls the liquid into this shape to minimize energy. This is also why light rainfall—known as drizzle with drop sizes less than 0.5 mm—consists of tiny, nearly perfect spheres falling through the air Physical Geography, PMF IAS, Hydrological Cycle, p.338.

Sources: Science, Class VIII NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.144; Physical Geography by PMF IAS, Hydrological Cycle (Water Cycle), p.338

3. Capillary Action and Liquid Rise (intermediate)

Capillary action is the spontaneous movement of a liquid through a narrow space, such as a thin tube or the pores of soil, often against the force of gravity. This phenomenon is driven by the delicate balance between two types of intermolecular forces: cohesive forces (the attraction between molecules of the same substance) and adhesive forces (the attraction between the liquid and the surrounding solid surface).

Whether a liquid rises or falls in a capillary tube depends on which force is stronger. When adhesive forces are greater than cohesive forces, the liquid "wets" the surface. For example, water wets glass because its attraction to the glass molecules is stronger than its internal attraction. This results in an acute contact angle (less than 90°) and the formation of a concave meniscus (a curve that dips in the middle). Conversely, if cohesive forces dominate, like in mercury, the liquid avoids the surface, resulting in an obtuse contact angle (greater than 90°) and a convex meniscus. In this case, the liquid level in the tube actually drops below the outside level.

Property	Wetting Liquid (e.g., Water)	Non-Wetting Liquid (e.g., Mercury)
Dominant Force	Adhesion > Cohesion	Cohesion > Adhesion
Contact Angle	Acute (< 90°)	Obtuse (> 90°)
Capillary Effect	Rise	Depression (Fall)
Meniscus Shape	Concave (U-shaped)	Convex (Dome-shaped)

In the real world, this mechanics concept has profound geographic and agricultural implications. In arid regions, high evaporation rates cause groundwater to be pulled upward through soil pores via capillary action Geography of India, Agriculture, p.67. As the water reaches the surface and evaporates, it leaves behind dissolved minerals, leading to the formation of saline and alkaline soils (locally called kallar or reh) and hard crusts known as hardpans Fundamentals of Physical Geography, Geomorphic Processes, p.45. Understanding the shape of the meniscus is even critical for basic science; when measuring liquids in a lab, one must read from the bottom of a concave meniscus for clear liquids, but the top for colored or non-wetting liquids to ensure accuracy Science Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.145.

Sources: Geography of India, Agriculture, p.67; Fundamentals of Physical Geography, Geomorphic Processes, p.45; Science Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.145

4. Fluid Pressure and Barometry (intermediate)

Fluid Pressure is fundamentally the force exerted by a fluid (liquid or gas) per unit area. In the context of our atmosphere, it is the weight of the air column stretching from the mean sea level to the very top of the atmosphere Physical Geography by PMF IAS, Pressure Systems and Wind System, p.304. This pressure is not a constant; it varies based on insolation (sunlight), temperature, and topography. For instance, at sea level, the standard atmospheric pressure is approximately 1013 millibars (mb) or 1 atmosphere (atm) Exploring Society: India and Beyond, Understanding the Weather, p.35.

To measure this pressure, we use an instrument called a Barometer. The traditional mercury barometer works by balancing the weight of a mercury column against the weight of the atmosphere. Mercury is chosen for two critical reasons: its high density (allowing for a manageable tube height) and its intermolecular properties. Unlike water, mercury has cohesive forces (attraction between similar molecules) that are significantly stronger than its adhesive forces (attraction to the glass). This means mercury does not "wet" the glass; it forms a convex meniscus and does not stick to the walls, ensuring an accurate and clean reading. In contrast, water would cling to the glass due to high adhesion, making precise measurement difficult.

As you ascend a mountain or fly in an airplane, the pressure drops because there is simply less air above you Certificate Physical and Human Geography, Weather, p.117. This drop is why high-altitude travelers must acclimatize to avoid altitude sickness. Because carrying a tall glass tube of liquid mercury is impractical for field researchers or climbers, we often use an Aneroid Barometer. This device contains a partially evacuated metal box that expands or contracts with external pressure changes, moving a dial to show the reading Certificate Physical and Human Geography, Weather, p.117.

Feature	Mercury Barometer	Aneroid Barometer
Mechanism	Liquid column height	Flexible vacuum chamber
Portability	Low (heavy/fragile)	High (compact/portable)
Accuracy	Very High	Moderate

Sources: Physical Geography by PMF IAS, Pressure Systems and Wind System, p.304; Exploring Society: India and Beyond, Understanding the Weather, p.35; Certificate Physical and Human Geography, Weather, p.117

5. Thermal Expansion and Fluid Properties (intermediate)

To understand why certain liquids 'wet' a surface while others don't, we must look at the particulate nature of matter. All matter is made of tiny particles held together by attractive forces. These forces are not just internal; they also act between the liquid and the container it touches. The physical state and behavior of these particles—whether they stay close or move past each other—depend heavily on their thermal energy Science, Class VIII, Particulate Nature of Matter, p.112.

The behavior of a liquid on a surface is determined by the tug-of-war between two specific types of attractive forces:

• Cohesive Forces: The attraction between similar molecules (e.g., water molecule to water molecule).
• Adhesive Forces: The attraction between dissimilar molecules (e.g., water molecule to glass molecule).

When a liquid like water or alcohol comes into contact with glass, the adhesive forces between the liquid and the glass are stronger than the cohesive forces within the liquid. This causes the liquid to spread out and 'wet' the surface. Conversely, Mercury—a unique metal that remains liquid at room temperature Science, Class VII, The World of Metals and Non-metals, p.43—behaves differently. Its internal cohesive forces are significantly stronger than its adhesion to glass. As a result, mercury avoids the glass, pulls inward to form spherical droplets, and does not 'wet' the vessel. This property, combined with its predictable thermal expansion (expanding when heated and contracting when cooled), makes it ideal for use in thermometers Certificate Physical and Human Geography, Weather, p.117.

Property	Wetting Liquids (e.g., Water)	Non-Wetting Liquids (e.g., Mercury)
Force Balance	Adhesion > Cohesion	Cohesion > Adhesion
Contact Angle	Acute (< 90°)	Obtuse (> 90°)
Surface Behavior	Spreads out / 'Clings'	Forms droplets / 'Avoids'

Sources: Science, Class VIII (NCERT), Particulate Nature of Matter, p.112; Science, Class VII (NCERT), The World of Metals and Non-metals, p.43; Certificate Physical and Human Geography (GC Leong), Weather, p.117

6. Contact Angle and Wetting behavior (exam-level)

To understand why some liquids 'stick' to surfaces while others bead up, we must look at the microscopic tug-of-war between two types of molecular attractions: cohesive forces (attraction between similar molecules within the liquid) and adhesive forces (attraction between liquid molecules and the solid surface). Wetting occurs when the adhesive forces are strong enough to overcome the internal cohesion of the liquid, causing it to spread across the surface. When you fill a glass tumbler with water, as described in Science - Class VII, Changes Around Us: Physical and Chemical, p.60, the water molecules are attracted to the glass molecules, leading to a 'wet' surface.

The physical manifestation of this balance is the Contact Angle (θ). This is the angle formed between the solid surface and the tangent to the liquid surface at the point of contact. We can categorize the behavior based on this angle:

Behavior	Force Relationship	Contact Angle (θ)	Example
Wetting	Adhesion > Cohesion	Acute (θ < 90°)	Water on Glass
Non-Wetting	Cohesion > Adhesion	Obtuse (θ > 90°)	Mercury on Glass

In the case of Mercury, the cohesive forces are exceptionally high. Instead of spreading out, mercury pulls itself into droplets to minimize contact with the glass, resulting in a contact angle of roughly 135° to 140°. Conversely, water and alcohol have high adhesion to glass, resulting in an acute angle. This behavior even affects how we see objects through these liquids; the curvature of the liquid surface (meniscus) can slightly alter the path of light, similar to how a lens or prism redirects light rays Science, Class X, The Human Eye and the Colourful World, p.165.

Sources: Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.60; Science , class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.165

7. Solving the Original PYQ (exam-level)

This question perfectly synthesizes the principles of molecular attraction you just mastered. To determine if a liquid 'wets' a surface, you must evaluate the competition between adhesive forces (the attraction between the liquid and the glass) and cohesive forces (the internal attraction between liquid molecules). As noted in NCERT Class 11 Physics, if the adhesion to the container is stronger than the internal cohesion, the liquid will spread across the surface. This physical behavior is represented by the contact angle; an acute angle indicates wetting, while an obtuse angle indicates a non-wetting liquid.

When applying this logic, Mercury stands out because its cohesive forces are exceptionally strong. These internal forces far outweigh the adhesive forces it shares with the glass molecules. Consequently, Mercury forms an obtuse contact angle (typically between 135° and 138°), causing it to bead up and curve away from the glass rather than spreading. This is why Mercury (C) is the correct answer; it is the only substance listed that maintains its spherical shape to minimize contact with the vessel walls, resulting in a convex meniscus.

UPSC often uses Water and Alcohol as distractors because they are the most common liquids we encounter. However, both possess strong adhesive forces with glass—driven by polar interactions and hydrogen bonding—which pull the liquid up the sides of the vessel to create a concave meniscus. Phenol also acts as a wetting agent in many chemical environments. The trap here is to confuse 'liquidity' with 'wetting.' Remember: wetting is not about being a fluid, but about the balance of attraction between the substance and its container.