The tendency of a liquid drop to contract and occupy minimum area is due to

1. Intermolecular Forces: Cohesion and Adhesion (basic)

To understand the chemistry of the world around us, we must first look at the "invisible glue" that holds matter together. All substances are made of tiny particles that exert forces on one another. As we see in Science, Class VIII, Particulate Nature of Matter, p.113, the behavior of these particles defines whether a substance is a solid or a liquid. While particles in a liquid can move past each other, they remain grouped together because of interparticle interactions Science, Class VIII, Particulate Nature of Matter, p.103.

These attractive forces are categorized into two types: Cohesion and Adhesion. To distinguish them, remember that Cohesion is the attraction between like molecules, while Adhesion is the attraction between unlike molecules. For example, when you use an "adhesive material" to fix a paper to a board Science, Class X, Magnetic Effects of Electric Current, p.196, you are utilizing the attraction between two different substances (the glue and the paper).

A remarkable consequence of cohesion is Surface Tension. In a body of liquid, a molecule in the center is pulled in every direction by its neighbors. However, a molecule at the surface has no liquid neighbors above it. Consequently, it is pulled strongly inward by the molecules below it. This inward pull creates a tension that makes the surface behave like a stretched elastic film, forcing the liquid to occupy the minimum possible surface area. Because a sphere is the geometric shape that has the smallest surface area for a given volume, small liquid droplets (like raindrops) naturally pull themselves into spherical shapes.

Sources: Science, Class VIII, Particulate Nature of Matter, p.103, 113; Science, Class X, Magnetic Effects of Electric Current, p.196

2. General Properties of Fluids: Density and Pressure (basic)

Welcome back! In our journey through applied chemistry, we must first understand the "personality" of fluids—a term that encompasses both liquids and gases. Two fundamental properties define how a fluid behaves in its environment: Density and Pressure. These aren't just abstract physics terms; they dictate why some things float, why others sink, and how our atmosphere stays glued to the planet.

Density (ρ) is the measure of how much matter is packed into a specific space, mathematically defined as mass per unit volume. Think of it as the "compactness" of a substance. In natural systems, density is rarely uniform. For example, as we look at the Earth's structure, we find that the density of material increases as we move from the surface toward the interior FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), The Origin and Evolution of the Earth, p.19. In everyday chemistry, density explains why oil sits on top of water; the oil molecules are less "crowded" than the water molecules, making the oil lighter for the same amount of space.

Pressure (P) is the force exerted by the fluid per unit of area. In any fluid body, whether it is the vast ocean or the air around us, pressure increases with depth FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), The Origin and Evolution of the Earth, p.19. This occurs because the fluid at the bottom must support the weight of all the fluid stacked above it. To give you a benchmark, the standard atmospheric pressure at sea level is approximately 1,013.25 mb Physical Geography by PMF IAS, Pressure Systems and Wind System, p.305. This pressure is what allows us to drink through a straw or explains why your ears "pop" when you drive up a mountain.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), The Origin and Evolution of the Earth, p.19; Physical Geography by PMF IAS, Pressure Systems and Wind System, p.305

3. Viscosity: Internal Friction in Liquids (intermediate)

When we think of friction, we usually imagine two solid surfaces rubbing against each other, like a brake pad against a wheel. However, fluids (liquids and gases) also experience a form of internal friction known as viscosity. Viscosity is the measure of a fluid's resistance to flow. Imagine a liquid as a series of thin layers sliding over one another. The layers closer to a fixed surface move slower, while those further away move faster; viscosity is the "drag" or force that resists this relative motion between the layers.

At the molecular level, viscosity is determined by intermolecular forces. In a highly viscous liquid like honey or motor oil, the molecules are large or have strong attractions to one another, making it difficult for them to slide past each other. Conversely, in a "thin" liquid like water or petrol, the internal friction is low, allowing for rapid flow. This concept is vital in everything from blood circulation in our bodies to the lubrication of industrial machinery. Interestingly, the environment also mirrors this; for instance, in the upper atmosphere, the low density of air results in significantly less friction, which allows jet streams to reach incredible velocities of up to 400 kmph or more Physical Geography by PMF IAS, Jet streams, p.386.

Temperature plays a crucial role in determining a liquid's viscosity. As a liquid is heated, the kinetic energy of its molecules increases, allowing them to overcome the attractive forces holding them together more easily. Consequently, the viscosity of most liquids decreases as temperature rises (think of how cold syrup is thick, but warm syrup pours easily). In contrast, the behavior of gases is different; their internal friction actually increases with temperature due to more frequent molecular collisions.

Sources: Physical Geography by PMF IAS, Jet streams, p.386

4. Buoyancy and Archimedes' Principle (intermediate)

Have you ever noticed how you feel significantly lighter when you're submerged in a swimming pool? This isn't an illusion; it is the result of a physical phenomenon known as buoyancy. When any object is placed in a fluid (a liquid or a gas), the fluid exerts an upward force on it. This force is what we call the buoyant force. While the history of measuring volumes dates back to ancient Mesopotamia—where scholars calculated the volume of water needed to cover fields—the specific relationship between this upward push and an object's weight was famously defined by the Greek scientist Archimedes Themes in world history, History Class XI, Writing and City Life, p.25.

Archimedes' Principle states that when an object is fully or partially immersed in a liquid, it experiences an upward force that is exactly equal to the weight of the liquid it displaces Science, Class VIII, Exploring Forces, p.76. Think of it this way: to take up space in the water, the object has to push some water out of the way. That "pushed away" water fights back with an upward force. Whether an object sinks or floats depends entirely on the battle between its own weight (pulling it down) and this buoyant force (pushing it up).

This principle is deeply tied to the concept of density (mass per unit volume). In applied chemistry and geography, we see that the density of water isn't always constant. For instance, salinity (salt content) and temperature significantly change water's density. Denser water (cold or salty) provides a stronger buoyant force because a specific volume of it weighs more than the same volume of fresh or warm water Physical Geography, PMF IAS, Ocean Movements, p.487. This is why it is much easier to float in the salty Dead Sea than in a freshwater lake. To compare substances easily, we use Relative Density, which is the ratio of a substance's density to the density of water at the same temperature Science, Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.141.

Sources: Science, Class VIII (NCERT 2025), Exploring Forces, p.76; Themes in world history, History Class XI (NCERT 2025), Writing and City Life, p.25; Science, Class VIII (NCERT 2025), The Amazing World of Solutes, Solvents, and Solutions, p.141; Physical Geography by PMF IAS, Ocean Movements, p.487

5. Capillarity and Surface Phenomena (intermediate)

6. Surface Tension and Energy Minimization (exam-level)

To understand why liquid droplets behave the way they do, we must look at the molecular world. In the "bulk" of a liquid, a molecule is surrounded by neighbors on all sides, experiencing equal attractive forces in every direction. However, a molecule at the surface is in a different predicament; it has neighbors below and to its sides, but none above it. This creates an unbalanced inward pull toward the center of the liquid. This net inward force makes the surface of the liquid behave like a stretched elastic membrane, a phenomenon we call surface tension.

Nature is fundamentally lazy—it always seeks the state of lowest energy. Molecules at the surface have higher potential energy because they lack the full set of stabilizing interactions that their neighbors in the bulk enjoy. To reach a stable, low-energy state, the liquid tries to have as few molecules at the surface as possible. This means the liquid will naturally contract to occupy the minimum possible surface area for its given volume. While we know that liquids generally take the shape of their container (Science VIII, Particulate Nature of Matter, p.104), when they are free-falling or in very small amounts where gravity is less dominant, they prioritize this area minimization.

Geometrically, the shape that provides the smallest surface area for a specific volume is a sphere. This is why raindrops, dew on a leaf, and beads of mercury are spherical. This internal "tug-of-war" is quite distinct from other properties like viscosity (resistance to flow) or pressure, which liquids exert in all directions against the walls of a container (Science VIII, Pressure, Winds, Storms, and Cyclones, p.85). Surface tension is specifically about the surface's tendency to shrink and minimize its energy footprint.

Sources: Science VIII, Particulate Nature of Matter, p.104; Science VIII, Pressure, Winds, Storms, and Cyclones, p.85

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamentals of intermolecular forces and cohesion, this question serves as the perfect application of those building blocks. Think back to our discussion on how molecules at the surface of a liquid lack neighbors above them, creating an unbalanced inward pull. This internal tension acts like a stretched elastic membrane, forcing the liquid to minimize its exposed surface. To arrive at the correct answer, (B) surface tension, you must follow the logic of energy minimization: because a sphere is the geometric shape that possesses the smallest surface area for a given volume, the liquid naturally contracts into a droplet to reach its most stable state.

As a student of the civil services, you must distinguish between various physical properties that UPSC often groups together to test your conceptual clarity. While (B) surface tension explains the shape and contraction of the drop, the other options describe entirely different behaviors. Viscosity is a trap designed to make you think of liquid movement; however, it refers to internal friction and resistance to flow, not surface area. Similarly, density (mass per unit volume) and vapour pressure (the pressure exerted by a vapor in equilibrium with its liquid phase) are intrinsic properties that do not govern the geometric contraction of the liquid mass.

By focusing on the mechanical effect of cohesive forces at the interface, as detailed in General Science NCERT, you can see that the tendency to occupy the minimum area is a unique signature of surface tension. In the exam, whenever you see a liquid acting like an "elastic film" or forming spherical shapes, let your mind immediately navigate to the inward pull of surface molecules.