Question

The shape of the meniscus of water in a glass tube is different from that of mercury in a glass tube. Why?

Short answer

The meniscus shape of water in a glass tube is concave due to its strong adhesion with the glass surface and cohesive forces between water molecules, which cause the water level to be higher at the edge. Conversely, the meniscus shape of mercury in a glass tube is convex because its strong cohesive forces significantly exceed the weak adhesion with the glass surface, leading to a lower mercury level near the edge. The difference in meniscus shapes results from the distinct adhesion, cohesion properties, and wetting characteristics of water and mercury on glass surfaces.

Step-by-step solution

Define the meniscus

A meniscus is the curved surface of a liquid in a container or another solid surface. The shape of the meniscus depends on the balance between adhesive forces and cohesive forces.

Define adhesion and cohesion

Adhesion is the force of attraction between unlike molecules, such as a liquid and a solid surface. Cohesion is the force of attraction between like molecules, such as liquid molecules.

Understand surface tension and wetting

Surface tension arises due to the imbalance of attractive forces between liquid molecules near the surface. It is responsible for the curved shape of the meniscus. Wetting refers to how well a liquid spreads over a surface. A liquid has good wetting characteristics if it easily spreads across a surface, like water on glass. Meanwhile, poor wetting occurs when a liquid does not spread easily on a surface, like mercury on glass.

Identify the adhesion and cohesion properties of water

Water molecules have strong cohesive forces, thanks to hydrogen bonding, which are even stronger than adhesive forces with glass. However, water is also highly polar and easily forms hydrogen bonds with the glass surface, creating strong adhesion between them.

Analyze the shape of the meniscus in a water-filled glass tube

Due to the strong adhesion properties, water spreads well on the glass surface. This leads to the formation of a concave meniscus (curve curving upwards) inside the glass tube as the adhesive forces between water and glass overcome the cohesive forces between the water molecules. The water level near the edge is higher than that of the center.

Identify the adhesion and cohesion properties of mercury

Mercury has very strong cohesive forces between its molecules due to metallic bonding, which are stronger than the adhesive forces between the glass surface and mercury atoms. Additionally, mercury is non-polar, creating weaker adhesive forces with glass compared to water.

Analyze the shape of the meniscus in a mercury-filled glass tube

Due to the weak adhesion properties, the mercury doesn't spread well on the glass surface. The cohesive forces between mercury molecules significantly exceed the adhesive forces with the glass surface. This leads to the formation of a convex meniscus (curvature curving downwards) inside the glass tube, as the mercury pulled inward. The mercury level near the edge is lower than that of the center.

Final Step: Compare the meniscus of water and mercury

In a glass tube, the meniscus of water is concave, with the water level higher at the edge, due to strong adhesion with the glass surface and cohesive forces between water molecules. On the other hand, the meniscus of mercury is convex, with the mercury level lower at the edge, due to significantly stronger cohesive forces overcoming the weak adhesion with the glass surface. This difference in meniscus shapes results from differences in adhesion, cohesion properties, and wetting of water and mercury on the glass surface.