Question

(a) Distinguish between adhesive forces and cohesive forces. (b) What adhesive and cohesive forces are involved when a paper towel absorbs water? (c) Explain the cause for the U-shaped meniscus formed when water is in a glass tube.

Short answer

(a) Adhesive forces are the forces of attraction between different materials, while cohesive forces are the forces of attraction within the same material. (b) In paper towel absorption, adhesive forces cause water to stick to the paper fibers, and cohesive forces cause water molecules to stay together and spread through the material. (c) A U-shaped meniscus forms in a glass tube due to the combination of adhesive forces causing the water to rise along the tube walls and cohesive forces pulling the water surface down in the middle.

Step-by-step solution

Part (a) Definitions of Adhesive and Cohesive Forces

Adhesive forces are the forces of attraction between molecules or particles of different types. They cause different materials to stick together, for example, water sticking to glass. Cohesive forces, on the other hand, are the forces of attraction between molecules or particles of the same type. They hold a substance together, for example, water molecules sticking to each other. In summary, adhesive forces cause interactions between different substances, while cohesive forces cause interactions within the same substance.

Part (b) Adhesive and Cohesive Forces in Paper Towel Absorption

When a paper towel absorbs water, both adhesive and cohesive forces are involved: 1. Adhesive forces: Water molecules are attracted to the fibers in the paper due to adhesive forces. This causes the water to stick to the paper towel and be absorbed. 2. Cohesive forces: Water molecules are also attracted to other water molecules due to cohesive forces. As a result, when water is absorbed into the paper towel, the cohesive forces make the water molecules stay together and spread through the material.

Part (c) Explanation for the U-shaped Meniscus in a Glass Tube

When water is in a glass tube, a U-shaped meniscus is formed due to the interactions between cohesive forces and adhesive forces. The adhesive forces between water molecules and the glass walls cause the water to "stick" to the glass and rise along the walls. As a result, the water level at the edge of the tube where it meets the glass is higher than the level in the middle. However, cohesive forces among water molecules also play a role in shaping the meniscus. Water molecules in the middle are more attracted to each other than the glass, pulling the water surface down and forming a concave curve. The combination of adhesive and cohesive forces results in a U-shaped meniscus when water is in a glass tube.

Key concepts

Adhesive Forces

When two different materials come into contact, the attraction that occurs between their particles is due to adhesive forces. These forces are the reason why substances such as paint, glue, or water can stick to other surfaces. In everyday life, we observe adhesive forces in action when water droplets cling to a window during rain or when a sticker adheres to a surface.

For example, when you spill water on a wooden table, the water doesn't just pool together but spreads out and wets the surface. This spreading out is due to the adhesive forces at play between the water molecules and the wood particles. Without adhesive forces, the world would be a very different place. We rely on these forces for numerous applications including medical bandages sticking to skin, ink from a pen being absorbed by paper, and much more.

Cohesive Forces

In contrast to adhesive forces, cohesive forces are the forces of attraction that occur between similar particles within a substance. Picture a drop of water on a waxed car hood, maintaining its rounded shape rather than spreading out. This is a perfect example of cohesive forces at work; the water molecules are more attracted to each other than they are to the waxed surface.

Cohesive forces are responsible for phenomena such as surface tension, which allows small insects to walk on water without sinking. These forces also contribute to the formation of droplets and the process of capillary action, where water can flow in narrow spaces without the assistance of external forces. Understanding cohesive forces help us to explain not only why liquids form droplets but also how certain materials can be strong and why liquids have specific boiling and melting points.

Absorption in Paper Towels

Paper towels are a common household item that demonstrate the interplay between adhesive and cohesive forces. The fibrous nature of paper towels means that they have a large surface area, which increases the potential for adhesive interactions with water molecules.

When a paper towel comes into contact with a liquid spill, the adhesive forces draw the liquid into the fibers of the towel. Yet, it's not just the adhesive force at play - as the liquid is absorbed, cohesive forces encourage the water molecules to continue following one another into the towel, thereby distributing the liquid throughout. This combination of forces allows paper towels to efficiently soak up spills and makes them a staple for cleaning tasks. Understanding this absorption principle has also guided the design of other materials, like diapers and wipes, which utilize similar concepts to achieve desired functionality.

U-shaped Meniscus

The U-shaped meniscus formed when water is placed in a tube is a fascinating result of the competing effects of adhesive and cohesive forces. The U-shape (concave) meniscus occurs because the water molecules are more strongly attracted to the glass walls of the tube (adhesive forces) than they are to the air above the water.

This causes the water near the glass to rise up along the sides. Meanwhile, in the center of the glass tube, cohesive forces pull the water molecules down slightly, since they are attracted to each other and not the air. This creates the characteristic U-shape, with higher water levels at the tube's edges and a lower level in the center. Different fluids and materials can produce different meniscus shapes, such as a convex meniscus when mercury is placed in a glass tube, illustrating how molecular interactions shape the behavior of liquids.