Question

You leave the house wearing a cotton T-shirt and are surprised by a sudden rainstorm. You notice that the water soaks into the T-shirt whereas it just beads up on your raincoat. In this example, are the adhesive forces or cohesive forces stronger between water and your T-shirt? Between water and your raincoat? Explain.

Short answer

Adhesive forces are stronger in the T-shirt; cohesive forces are stronger on the raincoat.

Step-by-step solution

Identify the forces involved

There are two main types of forces to consider: adhesive forces (the attraction between water molecules and the material) and cohesive forces (the attraction between water molecules themselves).

Analyze the T-shirt scenario

When water soaks into your cotton T-shirt, it means that adhesive forces between water molecules and the cotton are stronger than the cohesive forces. This is why the water spreads out and gets absorbed.

Analyze the raincoat scenario

When water beads up on the raincoat, it indicates that the cohesive forces between water molecules are stronger than the adhesive forces between the water and the raincoat material. This causes water to form droplets.

Compare the two scenarios

In the case of the T-shirt, adhesive forces are stronger than cohesive forces, leading to absorption. For the raincoat, cohesive forces are stronger than adhesive forces, leading to beading of water.

Key concepts

Adhesive Forces

Adhesive forces are the attractive interactions between different types of molecules. In our specific example of the cotton T-shirt and raincoat, adhesive forces play a critical role in determining how water interacts with these materials.

When you wear a cotton T-shirt and it gets soaked in a sudden rainstorm, it's due to strong adhesive forces between water molecules and the cotton fibers. These forces pull the water molecules into the fabric, causing the water to spread and get absorbed.

Adhesive forces depend on material properties like surface energy and chemical interactions. In your T-shirt scenario, the cotton fibers have polar groups that form hydrogen bonds with water molecules, making the water stick and spread out.

Understanding adhesive forces helps in designing and selecting materials for different applications, especially in clothing, where we may want fabrics that either absorb or repel water depending on the need.

Cohesive Forces

Cohesive forces are the attractive forces between molecules of the same substance. For water, cohesive forces are largely due to hydrogen bonding between water molecules.

When water beads up on your raincoat, it's because the cohesive forces between water molecules are stronger than the adhesive forces between the water and the raincoat material. Cohesive forces cause water molecules to stick together and form droplets or beads.

The raincoat material is usually made from hydrophobic (water-repellent) substances, making adhesive forces weaker. As a result, cohesive forces dominate, and water forms droplets that easily roll off.

Knowing about cohesive forces is essential for understanding the behavior of liquids on different surfaces. In this case, it helps in designing waterproof materials that prevent water penetration and keep you dry.

Material Properties

The behavior of materials when interacting with water is largely influenced by their specific properties. These properties determine whether a material is hydrophilic (water-attracting) or hydrophobic (water-repelling).

Your cotton T-shirt is hydrophilic. Cotton fibers have a high affinity for water due to their polar molecular structure. This makes the adhesive forces stronger, causing water to spread and absorb into the fabric.

On the other hand, your raincoat is designed to be hydrophobic. Materials such as polyester or nylon are often coated with a special water-repellent finish. This coating reduces adhesive forces and allows cohesive forces to dominate. Water thus beads up and rolls off the surface.

Understanding material properties helps in making informed choices about the right kind of fabric to wear in different weather conditions. It also plays a vital role in various engineering and industrial applications, ranging from designing comfortable clothing to creating effective waterproof products.