Which one of the following is not result of surface tension?

1. Intermolecular Forces: Cohesion and Adhesion (basic)

To understand how the world around us holds its shape, we must look at the microscopic level. All matter is composed of extremely small particles held together by interparticle forces of attraction. These forces act like an invisible glue; they are strongest in solids, slightly weaker in liquids, and nearly non-existent in gases Science, Class VIII, Chapter 7, p.113. When we study these forces in fluids, we categorize them into two distinct types based on which particles are being pulled together: Cohesion and Adhesion.

Cohesion is the force of attraction between molecules of the same substance. Think of it as the "internal bond" that keeps a substance unified. For example, a raindrop maintains its shape because the water molecules are pulling on one another, trying to stay together. In contrast, Adhesion is the force of attraction between molecules of different substances. This is what causes water to stick to the walls of a container or a glass slide Science, Class VIII, Chapter 7, p.104. When you use an adhesive like tape or glue to fix a paper to a board, you are literally utilizing these interparticle forces to bridge two different materials Science, Class X, Chapter 12, p.196.

Feature	Cohesion	Adhesion
Nature of Attraction	Between identical molecules (Like-to-Like)	Between different molecules (Like-to-Unlike)
Visual Result	Droplet formation, surface tension	Wetting of surfaces, sticking to walls
Example	Mercury atoms sticking to each other to form a ball	Water sticking to a glass window after rain

The interaction between these two forces determines how a liquid behaves when it touches a solid. If cohesive forces are stronger (like in Mercury), the liquid will bead up and avoid the surface. If adhesive forces are stronger (like water on clean glass), the liquid will spread out and "wet" the surface. This delicate balance is the foundation for more complex phenomena like surface tension and capillary action, which we will explore in the coming stages.

Sources: Science, Class VIII, Particulate Nature of Matter, p.113; Science, Class VIII, Particulate Nature of Matter, p.104; Science, Class X, Magnetic Effects of Electric Current, p.196

2. Introduction to Surface Tension and Surface Energy (basic)

Welcome back! Now that we understand the basic states of matter, let’s dive into a fascinating property of liquids: Surface Tension. Imagine the surface of a pond or a glass of water. It isn't just a boundary; it behaves like a stretched elastic membrane. This happens because of the behavior of molecules at the molecular level. Inside the liquid, a molecule is pulled equally in all directions by its neighbors. However, a molecule at the surface has no liquid neighbors above it. Consequently, it experiences a net inward pull toward the bulk of the liquid. This internal pull creates a state of tension, causing the liquid to contract and occupy the minimum possible surface area.

This tendency to minimize area is why small droplets of water or mercury are spherical—since a sphere is the geometric shape with the least surface area for a given volume. When you observe water in a measuring cylinder, you might notice the surface isn't perfectly flat but curved; this is called a meniscus Science, Class VIII NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.144. This curvature results from the tug-of-war between Cohesive forces (attraction between similar liquid molecules) and Adhesive forces (attraction between liquid molecules and the container wall).

Force Type	Description	Typical Result
Cohesion	Attraction between molecules of the same substance.	Liquid beads up (like mercury).
Adhesion	Attraction between molecules of different substances.	Liquid "wets" the surface and climbs (like water in glass).

It is also vital to distinguish surface tension from evaporation. While surface tension is an elastic property related to intermolecular "stickiness," evaporation is a phase transition. In evaporation, molecules gain enough kinetic energy to overcome these attractive forces entirely and escape into the air as vapour Science, Class VIII NCERT, Particulate Nature of Matter, p.105. Finally, the work required to increase the surface area of a liquid against these inward pulls is known as Surface Energy. Essentially, if you want to stretch the liquid's "skin," you have to put energy into the system.

Sources: Science, Class VIII NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.144; Science, Class VIII NCERT, Particulate Nature of Matter, p.105

3. Viscosity and Fluid Friction (intermediate)

When we think of friction, we usually imagine two solid surfaces rubbing against each other. However, friction also exists within fluids (liquids and gases). This internal resistance to flow is called viscosity. Think of it as "fluid friction." Just as it is harder to push a box across a carpet than a smooth floor, it is harder for layers of a highly viscous liquid to slide past one another.

From a molecular perspective, liquids consist of particles that are free to move but are still held together by interparticle forces of attraction Science, Class VIII. NCERT (Revised ed 2025), Chapter 7, p. 104. When a fluid flows, different layers move at different velocities. The layers closer to a solid surface (like the wall of a pipe) move slower than the layers in the center. The attractive forces between these layers act like a "tug-of-war," trying to prevent them from sliding apart. This is why some liquids, like honey or motor oil, flow slowly (high viscosity), while others, like water or alcohol, flow much faster (low viscosity).

Temperature plays a crucial role in determining viscosity. In most liquids, as the temperature rises, the kinetic energy of the molecules increases, allowing them to overcome the interparticle forces more easily. Consequently, the liquid becomes "thinner" or less viscous. This is why cold engine oil needs to warm up before it can effectively lubricate a car's moving parts. In contrast, gases behave differently: as temperature increases, gas molecules collide more frequently, actually increasing their internal friction and viscosity.

Feature	Solid Friction	Viscosity (Fluid Friction)
Surface of Action	Occurs between two solid surfaces in contact.	Occurs between internal layers of a fluid.
Effect of Area	Independent of the area of contact.	Dependent on the area of the layers and the velocity gradient.
Common Example	A wooden block sliding on a table.	Honey pouring slowly from a jar.

Sources: Science, Class VIII. NCERT (Revised ed 2025), Chapter 7: Particulate Nature of Matter, p.104

4. Phase Transitions: Evaporation and Vapour Pressure (intermediate)

To understand how water moves from our oceans into the atmosphere, we must look at the microscopic battle happening at the water's surface. Evaporation is a phase transition where a liquid turns into a gas at temperatures below its boiling point. Unlike boiling, which is a "bulk" phenomenon occurring throughout the liquid, evaporation is a surface phenomenon. At the surface, some molecules gain enough kinetic energy to overcome the attractive intermolecular forces of their neighbors and escape into the air as vapour Science, Class VIII, Chapter 7, p.105. This process is essentially a cooling process because the highest-energy molecules leave, reducing the average kinetic energy (temperature) of the remaining liquid.

A critical concept here is Vapour Pressure—the pressure exerted by the vapour molecules in equilibrium with the liquid. Think of it as the "escape tendency" of the molecules. The rate of evaporation is governed by several environmental factors:

• Temperature: As temperature rises, more molecules gain the energy needed to escape, increasing the vapour pressure Fundamentals of Physical Geography, Class XI, Water in the Atmosphere, p.86.
• Surface Area: Since evaporation happens only at the surface, a wider surface area provides more "exit points" for molecules.
• Humidity and Wind: If the air is already "saturated" with moisture (high relative humidity), there is less room for new vapour. Wind speeds up evaporation by blowing away the saturated layer of air near the surface and replacing it with drier (unsaturated) air Physical Geography by PMF IAS, Hydrological Cycle, p.328.
• Salinity: Interestingly, salt water evaporates about 5% slower than fresh water. This is because dissolved salts reduce the vapour pressure—the salt ions occupy surface space and "hold onto" water molecules, making it harder for them to bounce off Physical Geography by PMF IAS, Hydrological Cycle, p.329.

Finally, we must distinguish this from Boiling. A liquid boils when its internal vapour pressure equals the ambient atmospheric pressure. This explains why water can boil at room temperature if you significantly lower the surrounding pressure in a vacuum, or why it requires much higher temperatures to boil under the heavy CO₂ atmospheres of the early Earth Physical Geography by PMF IAS, Geological Time Scale, p.43.

Feature	Evaporation	Boiling
Location	Only at the surface	Throughout the entire liquid (bulk)
Temperature	Occurs at all temperatures	Occurs only at a specific boiling point
Speed	Slow process	Rapid process with bubble formation

Sources: Science, Class VIII NCERT (2025), Particulate Nature of Matter, p.105; Fundamentals of Physical Geography, Class XI NCERT (2025), Water in the Atmosphere, p.86; Physical Geography by PMF IAS, Hydrological Cycle (Water Cycle), p.328-329; Physical Geography by PMF IAS, Geological Time Scale The Evolution of The Earths Surface, p.43

5. Capillary Action and Meniscus Formation (intermediate)

To understand why liquids behave strangely in narrow tubes, we must look at the tug-of-war between two types of intermolecular forces: Cohesion (attraction between similar molecules, like water-to-water) and Adhesion (attraction between unlike molecules, like water-to-glass). When you pour a liquid into a container, these forces determine the shape of the surface, known as the meniscus. As noted in Science, Class VIII, NCERT (Revised ed 2025), Chapter 7, p. 104, while liquids have a definite volume, their particles are free to move and interact with the container walls.

If the adhesive forces are stronger than the cohesive forces, the liquid "wets" the surface and climbs up the walls, forming a concave meniscus (like water in glass). Conversely, if cohesive forces dominate, the liquid pulls away from the walls, creating a convex meniscus (typical of mercury). This curvature is critical for measurement accuracy. For colourless liquids like water, we always read the mark at the bottom of the meniscus, whereas for coloured liquids, we align with the top of the curve to ensure precision Science, Class VIII, NCERT (Revised ed 2025), Chapter 9, p. 144-145. To avoid errors, one must always take readings at eye level to eliminate parallax Certificate Physical and Human Geography, GC Leong, Weather, p. 115.

Capillary Action is the logical extension of this phenomenon. Because of surface tension, the curved meniscus creates a pressure difference across the liquid surface. In a very narrow tube (a capillary), this pressure difference is strong enough to pull the liquid upward against gravity (or push it down, in the case of mercury). This is why water rises higher in thinner tubes—a concept essential in everything from how plants draw water from roots to how a sponge absorbs a spill.

Feature	Concave Meniscus	Convex Meniscus
Force Balance	Adhesion > Cohesion	Cohesion > Adhesion
Appearance	Curves downward (U-shape)	Curves upward (inverted U)
Example	Water in a glass tube	Mercury in a glass tube

Sources: Science, Class VIII, NCERT (Revised ed 2025), Chapter 7: Particulate Nature of Matter, p.104; Science, Class VIII, NCERT (Revised ed 2025), Chapter 9: The Amazing World of Solutes, Solvents, and Solutions, p.144-145; Certificate Physical and Human Geography, GC Leong, Weather, p.115

6. Applications of Surface Tension in Daily Life (exam-level)

At its core, surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible. Think of the surface of water as a stretched elastic membrane. This occurs because molecules inside a liquid are pulled in all directions by neighboring molecules, but those at the surface have no neighbors above them. Consequently, they are pulled inward, creating an internal pressure that forces the liquid to contract. This is why small amounts of liquids, like mercury drops or falling raindrops, naturally form spherical shapes—since a sphere has the smallest surface area for a given volume Science, Class VIII NCERT (2025), Chapter 7, p.105.

In our daily lives, one of the most practical applications is cleansing action. Pure water has a high surface tension, which prevents it from spreading effectively into the tiny pores of fabric or around oily dirt. When we add soaps or detergents, they act as 'surfactants' that lower the surface tension of water. This allows the water to 'wet' the surface more effectively. Soap molecules form structures called micelles, where the hydrocarbon 'tail' attaches to oil/dirt and the ionic 'head' stays in the water, allowing the dirt to be pulled away and rinsed off Science, Class X NCERT (2025), Chapter 4, p.75.

Another fascinating manifestation is capillary action and the formation of a meniscus. When water is in a tube, the balance between cohesive forces (liquid-liquid attraction) and adhesive forces (liquid-container attraction) causes the surface to curve. This same tension allows water to rise through the narrow vessels of plants or a sponge to soak up a spill. It is also why insects like water striders can walk on a pond without sinking; their weight is distributed such that it doesn't break the 'elastic skin' of the water.

It is important to distinguish surface tension from evaporation. While both occur at the surface, they are driven by different physics. Surface tension is an elastic force minimizing area, whereas evaporation is a phase transition where molecules gain enough kinetic energy to break free into a gas state. In meteorology, we see the result of these forces in raindrops: if the drop is larger than 0.5 mm, it is called rainfall; if smaller, it is a drizzle Physical Geography by PMF IAS, Chapter 24, p.337.

Sources: Science, Class VIII NCERT (2025), Chapter 7: Particulate Nature of Matter, p.105; Science, Class X NCERT (2025), Chapter 4: Carbon and its Compounds, p.75; Physical Geography by PMF IAS, Chapter 24: Hydrological Cycle, p.337

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamentals of intermolecular forces, this question tests your ability to distinguish between the structural properties of a liquid and its phase transitions. Surface tension is essentially the "skin" effect caused by cohesive forces, where a liquid surface acts like a stretched elastic membrane to achieve the minimum possible surface area. As explored in Science, Class VIII. NCERT (Revised ed 2025), this inward pull is what dictates the physical geometry of liquids. When you evaluate options (B), (C), and (D), you will see they all describe the physical shaping or movement of a liquid bulk—whether it is forming a convex meniscus, climbing a tube via capillary action, or pulling itself into a spherical shape to minimize surface energy. These are direct physical manifestations of the liquid's surface acting under tension.

The correct answer is (A) Vapour formation above the liquid surface because it is a process of evaporation rather than a result of surface membrane strength. Reasoning through this, you must recognize that while surface tension is an attractive force that holds molecules together at the boundary, vapour formation occurs when individual molecules gain enough kinetic energy to overcome those attractions and escape into the air. UPSC often sets traps by grouping phenomena that happen "at the surface." However, as highlighted in Physical Geography by PMF IAS, vapour formation is primarily driven by temperature and vapour pressure gradients. To avoid this trap in the future, always ask: Is the liquid surface merely changing its shape, or are the molecules leaving the liquid phase entirely? If they are escaping the surface, the answer is usually related to thermodynamics, not the elastic property of surface tension.