MCQ on Surface Tension

Explanation: This question asks why separating two glass slides becomes difficult when a thin layer of water is present between them. The focus is on understanding the physical forces acting at the liquid interface. Surface tension arises due to cohesive forces between liquid molecules, which create a stretched, film-like surface. When water is trapped between two smooth surfaces, it spreads into a thin layer, increasing contact area and strengthening intermolecular attraction.

When the slides are pressed together, the water forms a uniform film. The molecules within this film attract each other strongly, creating a resistance to separation. Additionally, the air pressure outside the slides is higher than the pressure within the thin liquid layer, further pressing the slides together. As a result, pulling them apart requires overcoming both intermolecular cohesion and pressure differences, making the task difficult.

A simple analogy is two wet plates stacked together; they feel “stuck” because the water layer acts like a binding film. The effect is not due to glue but due to Molecular forces and pressure imbalance.

In summary, the resistance arises from the combined effect of intermolecular cohesion within the liquid film and external atmospheric pressure, which together hold the surfaces tightly in place.

Explanation: This question explores how a dense object like a needle can remain on the surface of water without sinking, despite being heavier than water. The concept involved is related to surface properties of liquids rather than buoyancy alone. Surface tension is caused by cohesive forces between liquid molecules, forming a thin, elastic-like layer at the surface.

When a needle is carefully placed on water, it does not immediately break the surface. Instead, the surface behaves like a stretched membrane that supports the needle’s weight. The molecules at the surface are more strongly attracted to each other than to the air, creating a tension that resists external force. If the needle is placed gently, this surface layer remains intact and holds it up.

An everyday example is small insects walking on water; they rely on the same principle. However, if the surface is disturbed or a substance like soap is added, this effect weakens and the object sinks.

In summary, the needle remains afloat because the liquid surface behaves like a stretched film due to cohesive Molecular forces, which can support small, carefully placed objects.

Explanation: This question asks about the standard unit used to quantify surface tension in scientific measurements. Surface tension represents force acting along the surface per unit length. It arises due to cohesive forces between molecules at the liquid’s surface, making the surface behave like a stretched elastic sheet.

To measure this property, we consider the force required to extend or break the surface over a specific length. Since force is measured in newtons and length in meters, the unit naturally combines these quantities. This reflects how much force is needed to act along a line on the liquid surface.

For instance, if you try to stretch a thin film of water, the resistance you feel per unit length corresponds to surface tension. It is not dependent on area or volume directly but on the linear dimension along which force acts.

In summary, the unit expresses how much force is applied along a unit length of the liquid surface, capturing the fundamental nature of this phenomenon.

Explanation: This question examines why insects like mosquitoes, which normally stand on water, fail to do so when kerosene is added. The key idea involves the role of surface tension in supporting lightweight Organisms on a liquid surface.

Under normal conditions, water has relatively high surface tension due to strong cohesive forces between its molecules. This creates a stable surface film that can support small insects. When kerosene spreads over the water, it disrupts these cohesive interactions and reduces the strength of the surface layer.

As the surface becomes weaker, it can no longer sustain the weight of mosquitoes. The liquid film breaks more easily under their legs, causing them to sink. This effect is not due to toxicity or smell but due to a physical change in the liquid’s surface properties.

An everyday example is adding soap to water, which similarly reduces surface tension and makes it easier for objects to sink.

In summary, the phenomenon occurs because the added substance weakens the liquid’s surface layer, preventing it from supporting the insects.

Explanation: This question focuses on the mechanism that allows kerosene to rise against gravity through the narrow spaces of a wick. The process involves intermolecular forces and the interaction between liquid and Solid surfaces.

When a wick is placed in kerosene, the liquid is drawn upward through tiny pores. This occurs because of adhesive forces between the liquid and the fibers of the wick, along with cohesive forces within the liquid itself. These forces create a pulling effect that allows the liquid to climb upward.

As the liquid rises, it continues to be drawn upward due to the continuous interaction between molecules and the narrow channels. The smaller the pores, the stronger this effect becomes. This upward movement continues until equilibrium is reached or the liquid is consumed by burning.

A similar effect can be seen when a paper towel absorbs water, drawing it upward without any external force.

In summary, the upward movement occurs due to intermolecular interactions between the liquid and the wick, enabling it to rise through narrow spaces.

Explanation: This question asks about the behavior of surface tension when a liquid approaches its critical temperature. At this temperature, the distinction between liquid and vapor phases disappears, and unique physical changes occur.

Surface tension exists because molecules at the surface experience unequal forces compared to those inside the liquid. However, as temperature increases, Molecular motion becomes more vigorous, reducing cohesive forces. At the critical temperature, the liquid and vapor phases become indistinguishable, meaning there is no defined surface.

Without a clear boundary, the concept of surface tension loses its meaning. The forces that create the surface layer vanish, and the liquid no longer exhibits the same behavior as before.

An analogy is the gradual fading of a boundary between two colors until they completely blend into one.

In summary, as the liquid reaches this special temperature, the surface disappears, and the associated surface forces cease to exist.

Explanation: This question explores how temperature affects the surface behavior of a liquid. Surface tension depends on the strength of cohesive forces between molecules at the surface.

As temperature increases, molecules gain kinetic energy and move more vigorously. This increased motion weakens the attractive forces holding the molecules together. As a result, the surface layer becomes less tightly bound and less resistant to deformation.

Because of this weakening, the surface tension gradually reduces with rising temperature. This effect continues until the liquid approaches its critical temperature, where the surface ceases to exist altogether.

A simple example is warm water, which feels less “tight” at the surface compared to cold water when touched lightly.

In summary, increasing temperature weakens intermolecular attraction, leading to a gradual reduction in the strength of the liquid’s surface layer.

Explanation: This question looks at why falling water droplets tend to take on a rounded shape. The explanation lies in how liquids behave under the influence of surface forces.

Molecules at the surface of a liquid experience inward attraction due to cohesive forces. This pulls the liquid into a shape that minimizes its surface area. Among all possible shapes, a sphere has the smallest surface area for a given volume, making it the most stable form.

As raindrops form and fall, these forces act uniformly in all directions, shaping them into nearly spherical forms. Although large drops may distort slightly due to air resistance, the tendency remains toward a rounded shape.

An example is small soap bubbles, which naturally form spheres for the same reason.

In summary, the rounded shape results from the liquid’s tendency to minimize surface area under the influence of internal Molecular attraction.

Explanation: This question focuses on identifying the type of force that acts between molecules of the same substance. Understanding intermolecular forces is essential for explaining many physical properties of Matter.

Molecules within a substance attract each other due to various forces, such as van der Waals interactions or hydrogen Bonding. When these attractions occur between identical molecules, they contribute to properties like surface tension, viscosity, and phase stability.

These forces help maintain the integrity of the substance, allowing it to exist as a liquid or Solid rather than dispersing completely. They are responsible for holding the molecules together in a cohesive manner.

An example is water molecules sticking together to form droplets rather than spreading infinitely.

In summary, the interaction between identical molecules plays a crucial role in maintaining the structure and physical behavior of a substance.

Explanation: This question examines the type of force that occurs between molecules of different materials. Such interactions are important in understanding processes like wetting, absorption, and capillary action.

When two different substances come into contact, their molecules may attract each other. The strength of this attraction determines how well one substance spreads over or sticks to another. For example, water interacting with glass shows strong attraction, allowing it to spread easily.

These interactions differ from those within the same substance and are essential in phenomena like liquid rising in narrow tubes or sticking to surfaces. They influence how substances mix or remain separate.

A practical example is paint adhering to a wall, which depends on these intermolecular attractions.

In summary, forces between different substances determine how they interact at the Molecular level, influencing spreading and attachment behaviors.

Explanation: This question explores why small droplets naturally adopt a spherical shape. The answer lies in how liquids respond to internal Molecular forces.

Molecules at the surface of a liquid experience a NET inward pull due to cohesion. This causes the liquid to contract and minimize its surface area. Since a sphere provides the least surface area for a given volume, it becomes the preferred shape.

As a result, small droplets, where surface forces dominate over gravity, tend to become spherical. This effect is more pronounced for tiny droplets where external forces like gravity are negligible.

An example is dew drops on leaves, which appear as small, round beads.

In summary, droplets form spheres because this shape minimizes surface energy, making it the most stable configuration under the influence of Molecular attraction.

Explanation: This question asks about the fundamental origin of surface tension in liquids. It arises due to the behavior of molecules at the liquid’s surface compared to those inside.

Inside the liquid, molecules are surrounded by others and experience balanced forces. At the surface, however, molecules are pulled inward because there are no molecules above them. This imbalance creates a NET inward force, forming a tight surface layer.

These inward forces result from cohesion between molecules, which binds them together. The surface behaves like a stretched film because of this imbalance and attraction.

A simple analogy is a group of people holding hands tightly at the edge of a crowd, creating a boundary that resists disturbance.

In summary, surface tension originates from unequal intermolecular forces acting on surface molecules, leading to a contracted and stable surface layer.

Explanation: This question asks why individual bristles of a paintbrush come together when dipped in water, instead of remaining spread apart. The phenomenon is related to intermolecular forces acting within the liquid surrounding the bristles.

When the brush is dry, the bristles are separate due to lack of significant forces binding them. Once dipped in water, a thin film forms around and between the bristles. The molecules in this liquid film attract each other strongly, creating a contracting effect on the surface.

As the liquid tries to minimize its surface area, it pulls the bristles inward, making them cling together. This effect is similar to how droplets tend to shrink into compact shapes. The force exerted by the liquid film effectively overcomes the tendency of the bristles to stay apart.

An everyday example is wet hair strands sticking together after a shower due to similar interactions.

In summary, the sticking of bristles occurs because the liquid film surrounding them contracts due to intermolecular attraction, pulling them into a compact arrangement.

Explanation: This question focuses on understanding the concept of Molecular range, which refers to the distance over which intermolecular forces are effective. It is an important idea in explaining how molecules interact within a substance.

Molecules exert forces on each other, but these forces are not effective over infinite distances. Instead, they act only within a limited region around each Molecule. Within this range, attractive forces help hold the substance together, while beyond it, the forces become negligible.

The concept explains why substances maintain structure in Solids and liquids but not at large separations. If molecules move beyond this range, they no longer influence each other significantly, leading to changes in physical state or behavior.

A simple analogy is two magnets: they attract each other only when brought close enough; beyond a certain distance, the attraction is no longer noticeable.

In summary, Molecular range defines the limited distance within which intermolecular forces act effectively, governing the cohesion and structure of Matter.