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

Adhesive forces are the attractive forces between molecules of different substances, while cohesive forces are the attractive forces between molecules of the same substance. In the case of a paper towel absorbing water, the adhesive forces are between the water molecules and the paper towel fibers, and the cohesive forces are between the water molecules themselves. The U-shaped meniscus formed when water is in a glass tube is due to the balance between these adhesive and cohesive forces, with the adhesive forces between the water molecules and the glass being stronger than the cohesive forces between the water molecules.

Step-by-step solution

Definition of Adhesive Forces

Adhesive forces are the forces of attraction between molecules from two different substances. These forces cause two different materials to stick together.

Definition of Cohesive Forces

Cohesive forces are the forces of attraction between molecules of the same substance. These forces hold the molecules together within a single material.

Forces Involved in the Absorption of Water by a Paper Towel

When a paper towel absorbs water, both adhesive and cohesive forces are involved. The adhesive forces are between the water molecules and the paper towel fibers, which cause the water to be attracted to and stick to the paper towel. The cohesive forces are between the water molecules themselves, which hold the water together as it is absorbed by the paper towel.

Cause of the U-Shaped Meniscus When Water is in a Glass Tube

When water is in a glass tube, a U-shaped meniscus forms due to the interplay between adhesive and cohesive forces. Adhesive forces between the water molecules and the glass cause the water to be attracted to the glass, resulting in the water climbing up the sides of the tube. The cohesive forces between the water molecules, on the other hand, pull the water together, maintaining its curved shape within the tube. This balance between adhesive and cohesive forces creates the U-shaped meniscus. In this case, the adhesive forces between the water molecules and the glass are stronger than the cohesive forces between the water molecules themselves.

Key concepts

Adhesive Forces

Adhesive forces are the attractive forces that occur between molecules of different substances. This kind of force is pivotal in enabling substances like glue to stick objects together.
When you think about using tape or glue, it is the adhesive forces at work that bind materials together. These forces are responsible for a variety of everyday phenomena, such as paint sticking to a wall or ink adhering to a page.

• The strength of adhesive forces depends on the nature of the substances involved.
• For example, adhesive forces are essential in medical applications where bandages need to adhere to skin.

Understanding adhesive forces helps us comprehend why some materials mix well or stick together.

Cohesive Forces

Cohesive forces refer to the forces of attraction that occur between molecules of the same substance. These forces are what keep a droplet of water intact or maintain the shape of liquid bodies.
Think of cohesive forces as the internal glue holding the molecules of a single substance together. Without these forces, each molecule would drift aimlessly apart in the liquid.

• Cohesion is what allows water to form droplets on a leaf or rain to bead up on a car window.
• Strong cohesive forces lead to a surface tension effect, where the surface of a liquid acts like an elastic "skin."

The concept of cohesive forces is fundamental in explaining phenomena such as surface tension and the spherical shape of liquid droplets.

Capillary Action

Capillary action is the ability of a liquid to flow in narrow spaces without external assistance, sometimes even against gravity. This can be observed when a paper towel absorbs water.
The process involves both adhesive and cohesive forces. Adhesive forces between the liquid and the solid surface assist the liquid in climbing up, while cohesive forces within the liquid maintain the liquid column's integrity.

• In natural environments, capillary action is seen when plant roots draw water from the soil.
• It's also the reason why a thin tube dipped into water will show the liquid climbing up within the tube.

Capillary action is a critical concept in understanding how plants hydrate and in designing tools like microfluidic devices in labs.

Meniscus Formation

A meniscus is the curve seen at the top of a liquid in response to its container. Meniscus formation results from the balance between adhesive and cohesive forces.
When water is in a glass tube, the adhesive forces between water and glass are stronger than the cohesive forces among water molecules. This causes the water to climb up the sides, creating a U-shaped curve, known as a concave meniscus.

• An inverted U-shape, or convex meniscus, occurs with liquids like mercury, where cohesive forces are stronger, pushing the liquid up in the center.
• Understanding meniscus formation is crucial in accurately measuring liquid volumes in laboratory settings.

Meniscus formation not only affects measurements but also plays a role in processes like plant transpiration and liquid containment methodologies.