When a water droplet is placed on a freshly cut piece of wood, it spreads out to form a thin layer because the wood is

1. Analyze the Question

The question asks why a water droplet spreads out into a thin layer when placed on a freshly cut piece of wood. We need to identify the physical force or property responsible for this behavior.

2. Explain the Concept

This phenomenon is governed by two fundamental forces in fluid mechanics:

• Cohesion: This is the force of attraction between molecules of the same substance (e.g., water molecules sticking to other water molecules).
• Adhesion: This is the force of attraction between molecules of different substances (e.g., water molecules sticking to the surface of the wood).

When a liquid spreads out on a surface (wetting the surface), it means the adhesive forces between the liquid and the solid surface are stronger than the cohesive forces within the liquid itself. If the cohesive forces were stronger, the water would bead up into a sphere to minimize contact with the surface.

3. Show the Solution

• Step 1: Observe the behavior of the water. Since the water spreads into a thin layer, it is "wetting" the wood.
• Step 2: Relate the behavior to the forces. Wetting occurs when the attraction between the water molecules and the wood molecules (adhesion) is greater than the attraction between the water molecules themselves (cohesion).
• Step 3: Evaluate the options. • A & C: Equilibrium refers to a state of balance, which does not explain the spreading behavior. • B: If the wood were cohesive to water, it would imply the wood is pulling water molecules toward other water molecules, which is incorrect. • D: Because the wood attracts the water molecules, we describe the wood as being adhesive to the water.

4. Identify the Answer

The correct option is D. The wood is adhesive to the water, meaning the adhesive forces between the wood and water are strong enough to overcome the cohesive forces of the water, causing it to spread.

5. Tip

Memory Trick: Think of Adhesion as Attraction between Another substance and the liquid. Think of Cohesion as Connection between the Came (same) substance. If the liquid spreads, Adhesion wins!