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 difference in the shapes of the meniscus of water and mercury in a glass tube is due to the variation in cohesive and adhesive forces acting at the liquid-glass interface. Water forms a concave meniscus because its adhesive forces with glass are stronger than its cohesive forces, causing water molecules to be more attracted to the glass than to each other. On the other hand, mercury forms a convex meniscus due to its stronger cohesive forces compared to its adhesive forces with glass, making mercury molecules more attracted to each other than to the glass surface.

Step-by-step solution

Understanding cohesive and adhesive forces

Cohesive forces are the forces of attraction between molecules of the same substance. In the case of liquids, it is the intermolecular forces holding the liquid together. Adhesive forces, on the other hand, are the forces of attraction between molecules of different substances, such as the liquid and the solid surface with which it is in contact.

Meniscus of water in a glass tube

In the case of water and glass, the adhesive forces between water molecules and glass molecules are stronger than the cohesive forces between water molecules. This means that water molecules are attracted more to the glass surface than to each other. As a result, the water molecules in contact with the glass tube are pulled upwards, creating a concave meniscus (curving upwards at the edges).

Meniscus of mercury in a glass tube

Mercury has stronger cohesive forces compared to its adhesive forces with glass. This means that mercury molecules are more attracted to each other than to the glass molecules. As a result, mercury tends to minimize its contact with the glass surface, leading to a convex meniscus (curving downwards at the edges).

Comparison between water and mercury meniscus

The difference in the shapes of the meniscus of water and mercury in a glass tube arises due to the variation in the cohesive and adhesive forces acting at the liquid-glass interface. Water has a concave meniscus because its adhesive forces with glass are stronger than its cohesive forces, while mercury has a convex meniscus due to its stronger cohesive forces compared to its adhesive forces with glass.

Key concepts

Cohesive Forces

Cohesive forces are the forces of attraction between molecules of the same substance. This means they act to hold the liquid together. The stronger these forces are, the more tightly the molecules in the liquid stick to each other. For example, in mercury, cohesive forces are quite strong. This causes the mercury molecules to pull together, forming droplets or beads on surfaces. Cohesive forces are critical in determining how a liquid will behave in different containers.

• Strong cohesive forces lead to liquids forming compact shapes.
• They influence whether a liquid spreads out or forms beads.

Understanding cohesive forces helps explain phenomena like surface tension, which is essential in a wide range of natural and industrial processes.

Adhesive Forces

Adhesive forces are the forces of attraction between molecules of different substances. In our context, this often involves a liquid and a solid surface, such as water in contact with glass. These forces determine how the liquid interacts with surfaces it touches. When adhesive forces are stronger than cohesive forces, the liquid tends to spread over the surface.

• This behavior is seen with water in a glass tube.
• Water's adhesive forces with glass molecules are stronger, causing the water to climb up the glass.

In practical terms, understanding adhesive forces is important in applications like painting or coatings, where strong adhesion to surfaces is required for durability and effectiveness.

Concave Meniscus

A concave meniscus is formed when the adhesive forces between a liquid and its container walls are stronger than the cohesive forces within the liquid itself. This results in the liquid being more attracted to the container walls, creating a downward curve or a dip in the surface of the liquid where it meets the walls. Water in a glass tube is a classic example.

• The adhesion between water molecules and glass is stronger than the cohesion among water molecules.
• This pulls the water up along the sides of the container.

Understanding the formation of concave menisci is essential in fields like chemistry and biology. This knowledge helps explain how liquid levels are measured accurately in laboratory equipment such as pipettes and burettes.

Convex Meniscus

A convex meniscus occurs when cohesive forces within the liquid are stronger than adhesive forces with the container walls. As a result, the liquid pulls away from the container walls, creating a bulging shape upwards. Mercury is a substance that typically exhibits a convex meniscus in a glass tube.

• Mercury's cohesive forces are stronger than its adhesive forces with glass.
• This causes the surface to pull inward, minimizing contact with the container walls.

Recognizing convex menisci is important for understanding how liquids behave in tubes and containers. It also has practical implications in devices like barometers and thermometers, where accurate readings depend on the shapes of the menisci.