Question

A liquid wets a solid completely. The meniscus of the liquid in a sufficiently long tube is (A) Flat (B) Concave (C) Convex (D) Cylindrical

Short answer

The shape of the meniscus when a liquid wets a solid completely is (B) Concave. In this case, the adhesive forces between the liquid and the solid are stronger than the cohesive forces within the liquid, causing the liquid to rise against gravity, resulting in a concave meniscus.

Step-by-step solution

Understanding surface tension

Surface tension is a property of liquids that causes the surface of the liquid to behave like a stretched elastic membrane. This results from the imbalance of forces acting on the surface of the liquid, as the molecules at the surface of a liquid experience a net inward force due to the attractive forces between the liquid molecules. This force causes the surface of the liquid to minimize its area.

Wetting of a solid surface

Wetting occurs when a liquid spreads out over a solid surface and forms an interface between the liquid and the solid. The degree to which a liquid wets a solid surface depends on the balance between the cohesive forces within the liquid (liquid-liquid interactions) and adhesive forces between the liquid and solid (liquid-solid interactions). When the adhesive forces are stronger than the cohesive forces, the liquid will spread out completely over the surface and wet the solid.

Meniscus formation

A meniscus is the curve at the surface of a liquid in a container, resulting from the balance between the adhesive forces (between the liquid and the container's wall) and the cohesive forces (among the liquid molecules).

Determining the shape of the meniscus

In the case of a liquid that wets a solid completely, the adhesive forces between the liquid and the solid are stronger than the cohesive forces within the liquid. As a result, the liquid will spread and rise against gravity to maximize the contact area with the solid. This causes the meniscus to be concave, as the liquid is pulled up by the adhesive forces and held down by the cohesive forces. Based on this analysis, the correct answer is: (B) Concave

Key concepts

Surface Tension

Surface tension is a fascinating property of liquids that can often be observed in everyday life. It acts like a thin elastic sheet covering the surface of a liquid due to the intermolecular forces. These forces make the molecules on the surface experience a net inward pull, causing them to stick together closely.
As a result, liquids tend to form droplets with a minimized surface area, like raindrops forming on leaves. This is because surface tension ultimately seeks to minimize the amount of surface exposed to an air-solid or air-liquid interface. It explains why small insects can walk on water and how it creates that shimmering film over water in a glass.

• Caused by an imbalance of molecular forces.
• Makes the liquid surface behave like a stretched membrane.
• Promotes the formation of droplets and bubbles.

Surface tension is crucial in understanding how and why liquids form various shapes, including the meniscus seen in containers like test tubes.

Cohesive Forces

Cohesive forces are the attractive forces that hold molecules within the same substance together. In a liquid, these forces keep the molecules tightly bound, affecting how they behave in different situations.
For example, when you see water droplets, it's the cohesive forces at work, holding the water molecules together to form a drop. The strength of these forces determines many properties of liquids, like boiling and melting points.

• Keep the substance molecules together.
• Lead to the formation of droplets.
• Impact the liquid's physical characteristics.

Cohesive forces compete with adhesive forces during phenomena such as wetting and meniscus formation. When these forces are higher than the adhesive forces, a liquid will not spread out completely over a surface.

Adhesive Forces

Adhesive forces occur when molecules of different substances are attracted to each other. They play a key role in the interactions between liquids and solids.
For instance, when water climbs up a thin tube or spreads on a glass surface, adhesive forces are at play. These forces help the molecules of the liquid adhere to surfaces, influencing how it spreads.

• Cause liquid to stick to solid surfaces.
• Can overpower cohesive forces, leading to wetting.
• Important in processes like painting and coatings.

In the context of a meniscus, strong adhesive forces between the liquid and the container contribute to the liquid 'climbing' and creating a concave shape.

Wetting of Surfaces

Wetting of surfaces is a phenomenon where a liquid spreads across or adheres to a solid, creating a contact interface. This is guided by the balance between cohesive forces within the liquid and adhesive forces between the liquid and the solid.
When adhesive forces are stronger than cohesive forces, the liquid will wet the surface, spreading across it. This wetting creates a concave meniscus, often seen in test tubes with water. Such wetting is pivotal in processes such as painting, gluing, and even the operation of detergents.

• Dependent on the balance of cohesive and adhesive forces.
• Crucial in practical applications like glue and cleaning.
• Leads to different meniscus shapes depending on the liquid and surface.

Understanding wetting helps explain why some surfaces get soaked while others repel water, like a waterproof jacket.