Hair of a shaving brush cling together when the brush is removed from water due to

1. Foundations of Matter: Cohesion and Adhesion (basic)

To understand the mechanics of the world around us, we must start at the very foundation: matter. At the microscopic level, all matter is composed of extremely small particles that are constantly interacting. These particles don't just float aimlessly; they are held together by interparticle forces of attraction Science, Class VIII, NCERT (Revised ed 2025), Chapter 7, p.113. When we talk about how things stick together or fall apart, we are essentially talking about two specific types of these attractive forces: Cohesion and Adhesion.

Cohesion is the force of attraction between particles of the same substance. Think of it as the internal "glue" that keeps a raindrop spherical or allows mercury to bead up on a surface. Because these forces are strongest in solids and slightly weaker in liquids, solids can maintain a fixed shape while liquids possess a definite volume but flow Science, Class VIII, NCERT (Revised ed 2025), Chapter 7, p.113. On the other hand, Adhesion is the force of attraction between particles of different substances. This is why water "wets" a surface or sticks to the walls of a container, sometimes causing the measured volume to appear slightly less when poured into a new vessel Science, Class VIII, NCERT (Revised ed 2025), Chapter 7, p.104.

Force Type	Interaction	Real-world Example
Cohesion	Between similar molecules	Water molecules forming a droplet.
Adhesion	Between different molecules	Water sticking to your skin after a shower.

These forces play a dramatic role in a phenomenon called Surface Tension. Consider a shaving brush: when it is submerged in water, the hairs move freely because they are surrounded by liquid. However, the moment you pull the brush out, the hairs suddenly cling together. This happens because a thin film of water forms between the hairs. Due to cohesive forces, the water molecules try to pull toward each other to minimize their surface area, acting like a tight elastic skin that drags the hairs inward. This is a classic example of how microscopic molecular attractions create macroscopic physical changes.

Sources: Science, Class VIII, NCERT (Revised ed 2025), Particulate Nature of Matter, p.113; Science, Class VIII, NCERT (Revised ed 2025), Particulate Nature of Matter, p.104

2. Viscosity: Internal Friction in Fluids (intermediate)

When we think of friction, we usually imagine two solid surfaces rubbing against each other, like your shoes on a pavement. However, friction is not limited to solids. Viscosity is essentially the "internal friction" of a fluid (which includes both liquids and gases). It represents the resistance that a fluid offers to flow. Imagine trying to pour honey versus pouring water; the honey flows much more slowly because it has a higher viscosity. This happens because the layers of the fluid exert a dragging force on one another as they move at different speeds. As noted in Science, Class VIII, NCERT (Revised ed 2025), Exploring Forces, p. 68, liquids and gases exert this force of friction on any object moving through them, a phenomenon often referred to as drag.

To move efficiently through these fluids, the shape of an object matters immensely. This is why aeroplanes, ships, and even aquatic animals have streamlined bodies to minimize the fluid friction or drag Science, Class VIII, NCERT (Revised ed 2025), Exploring Forces, p. 68. In the atmosphere, this friction isn't just a lab concept; it affects our global weather. For instance, the irregularities of the Earth's surface create friction that resists wind movement, an effect that can extend up to 1-3 km into the sky Physical Geography, PMF IAS, Pressure Systems and Wind System, p. 307. Interestingly, because the sea surface is much smoother than land, friction over the ocean is minimal, allowing winds to maintain higher speeds.

It is important to distinguish viscosity from surface tension. While viscosity is about the internal resistance to flow throughout the bulk of the liquid, surface tension is a force acting specifically at the surface to minimize area. A classic observation is a shaving brush: while submerged, the hairs stay apart because they are surrounded by water. Once removed, a thin film forms, and surface tension pulls the hairs together to reduce the surface area. While viscosity affects how much water is "carried" out on the brush, the actual clinging effect is a result of surface tension trying to contract the liquid surface.

Feature	High Viscosity (e.g., Honey)	Low Viscosity (e.g., Water)
Flow Rate	Slow and resistant	Fast and easy
Internal Friction	Very High	Low
Molecular Interaction	Strong internal "drag" between layers	Weak internal "drag" between layers

Sources: Science, Class VIII, NCERT (Revised ed 2025), Exploring Forces, p.68; Physical Geography, PMF IAS, Pressure Systems and Wind System, p.307

3. Elasticity: Response to Deformation (intermediate)

Elasticity is the property of a material that allows it to return to its original shape and size after the external force causing the deformation is removed. At its core, this phenomenon is a tug-of-war at the atomic level. In solids, the constituent particles are closely packed and held together by very strong interparticle interactions Science, Class VIII (2025), Particulate Nature of Matter, p.113. When you pull or compress an object, you are essentially trying to move these particles away from their stable, fixed positions. The internal forces between the particles act like tiny, invisible springs, pushing or pulling back to restore the original arrangement.

A practical application of this concept is the spring balance. When an object is hung from the hook, the gravitational force (weight) pulls the spring downwards, causing it to stretch. This stretching is a form of deformation. Because the spring is elastic, the amount it stretches is directly related to the force applied, allowing us to measure weight in newtons Science, Class VIII (2025), Exploring Forces, p.73. However, elasticity has its limits. If you stretch a material too far—beyond its elastic limit—the particles are displaced so significantly that they cannot return to their original spots, leading to permanent (plastic) deformation.

It is important to distinguish how different states of matter respond to such forces. While gases are highly compressible because their particles are far apart, solids are nearly incompressible. Because the particles in solids are already very close to each other, applying pressure typically results in negligible changes to their density Science, Class VIII (2025), The Amazing World of Solutes, Solvents, and Solutions, p.148. This rigidity is what makes solid structures reliable for engineering, but their underlying elasticity is what prevents them from snapping the moment a small force is applied.

Property	Elasticity	Plasticity
Response to Force	Temporary deformation; object snaps back.	Permanent deformation; object stays reshaped.
Internal Particles	Restoring forces pull particles back to original positions.	Particles slide into new positions and stay there.

Sources: Science, Class VIII (2025), Particulate Nature of Matter, p.113; Science, Class VIII (2025), Exploring Forces, p.73; Science, Class VIII (2025), The Amazing World of Solutes, Solvents, and Solutions, p.148

4. Capillarity and the Meniscus (exam-level)

To understand how liquids behave in narrow spaces or against surfaces, we must look at the tug-of-war between two internal forces: Cohesion (the attraction between similar molecules, like water to water) and Adhesion (the attraction between different molecules, like water to glass). When a liquid is placed in a container, it exerts pressure in all directions, including against the walls (Science, Class VIII, Chapter 7, p.85). However, right at the boundary where the liquid meets the solid, these forces create a curved surface known as a Meniscus.

Whether a liquid 'climbs' a wall or 'ducks' away from it depends on which force is stronger. This phenomenon leads to Capillarity—the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity. We see this in nature when sap rises in trees or when a paper towel absorbs a spill.

Meniscus Type	Primary Force	Example	Behavior
Concave	Adhesion > Cohesion	Water in glass	Liquid 'wets' the surface and rises.
Convex	Cohesion > Adhesion	Mercury in glass	Liquid pulls away and its level falls.

A classic application of these principles is observed with a shaving brush or paintbrush. When the brush is fully submerged, the hairs stay apart because they are surrounded by water. However, the moment you withdraw the brush, the hairs cling together. This happens because a thin film of water forms between the hairs, and surface tension—driven by cohesive forces—attempts to contract and minimize the surface area of that film (Science, Class VIII, Chapter 7, p.105). This contraction pulls the individual hairs toward each other, demonstrating the powerful 'clinging' effect of capillary attraction.

Sources: Science, Class VIII, NCERT (Revised ed 2025), Chapter 7: Particulate Nature of Matter, p.85, 105

5. Surface Tension: Minimizing Surface Area (exam-level)

At the heart of many liquid behaviors is a phenomenon called Surface Tension. Think of a liquid's surface not just as a boundary, but as a stretched elastic membrane. This occurs because of cohesive forces—the internal attraction between similar molecules. While a molecule deep inside a liquid is pulled equally in all directions by its neighbors, a molecule at the surface has no neighbors above it. Consequently, it experiences a net inward pull toward the bulk of the liquid. This inward force creates a state of tension that makes the liquid surface want to occupy the smallest possible area. Because of this drive to minimize surface area, small amounts of liquid naturally form spherical shapes (like raindrops or dew), as a sphere provides the least surface area for a given volume. While we know that liquids have a definite volume but no fixed shape Science, Class VIII. NCERT(Revised ed 2025), Chapter 7, p.104, surface tension acts as an invisible hand that tries to 'shrink-wrap' the liquid into its most compact form. A classic application of this principle is seen with a shaving brush or paintbrush. When the brush is submerged in water, the hairs stay apart and move freely because they are surrounded by water from all sides. However, the moment you lift the brush out, a thin film of water remains trapped between the bristles. To minimize the surface area of this water-air interface, surface tension pulls the film inward, dragging the flexible hairs toward each other. This is why the bristles cling together tightly only when removed from the water.

Sources: Science, Class VIII. NCERT(Revised ed 2025), Chapter 7: Particulate Nature of Matter, p.104-105

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

To understand the applications of surface tension, we must first look at the 'tug-of-war' happening at the molecular level. Molecules inside a liquid are pulled in all directions by their neighbors, but those at the surface have no neighbors above them. This creates a net inward pull, making the surface behave like a stretched elastic membrane trying to contract to the smallest possible area. A fascinating example of this is seen with a shaving brush. When the brush is underwater, the hairs stay spread out because they are completely surrounded by water. However, the moment you pull the brush out, the hairs suddenly cling together. Why? As the brush leaves the water, a thin film of liquid forms between the individual hairs. Surface tension acts to minimize the surface area of this film, pulling the hairs toward each other to reduce the interface between the water and the air Science, Class VIII (2025 ed.), Particulate Nature of Matter, p.105. This same principle explains why raindrops are spherical; a sphere is the shape that offers the minimum surface area for a given volume. We also manipulate surface tension for hygiene. Pure water has a high surface tension, meaning it tends to 'bead up' rather than soak into the tiny pores of dirty clothes. To fix this, we use soaps and detergents. These substances are surfactants (surface-active agents) that lower the surface tension of water. Soap molecules consist of a long hydrocarbon chain (hydrophobic) and an ionic end (hydrophilic) Science, Class X (2025 ed.), Carbon and its Compounds, p.75. By reducing the 'skin-like' tension of the water, these molecules allow the liquid to 'wet' the fabric more effectively and form micelles that trap oily dirt, pulling it away into the water Science, Class X (2025 ed.), Carbon and its Compounds, p.76. In the natural world, plants and insects also rely on these mechanics. The Drosera (Sundew) plant secretes sticky droplets that glisten like dew. When an insect touches these drops, the combination of surface tension and adhesion makes the fluid act like a trap, ensnaring the prey so the plant can absorb its nutrients Environment (Shankar IAS), Plant Diversity of India, p.198.

Sources: Science, Class VIII (NCERT 2025 ed.), Particulate Nature of Matter, p.105; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.75; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.76; Environment, Shankar IAS Academy (10th ed.), Plant Diversity of India, p.198

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental properties of matter, you can see how cohesive forces manifest in real-world scenarios. This question tests your ability to apply the concept of molecular attraction at a liquid-air interface. While the brush is submerged, the hairs are surrounded by water molecules on all sides, resulting in no net force pulling them together. However, the moment the brush is removed, a thin film of water remains between the bristles, creating a new interface with the air. Here, the molecules at the surface experience an inward pull, a phenomenon you learned as surface tension.

To arrive at the correct answer, (B) surface tension, imagine the water film acting like a stretched elastic membrane. Why does this happen? Because the liquid naturally seeks to minimize its surface area to reach its lowest energy state. As this film contracts, it exerts a lateral force that pulls the flexible hairs of the brush toward each other, causing them to "cling." This is a classic demonstration of capillary attraction driven by the tendency of liquids to shrink their exposed surface, as discussed in Science, Class VIII, NCERT (Revised ed 2025).

UPSC often includes distractors to test the depth of your conceptual clarity. You might be tempted by viscosity, but remember that viscosity represents a fluid's resistance to flow or internal friction, not the attractive force at an interface. Similarly, friction is a force that opposes relative motion between surfaces and cannot explain why the hairs move toward each other initially. While elasticity is a property of the brush bristles themselves, it would actually cause the hairs to spring apart to their original shape if the water weren't present. It is the external force of the contracting water film that overcomes the bristles' natural state.