The correct option is **B. Surface tension allows droplets to maintain a spherical shape due to cohesive forces between liquid molecules.** ### Detailed Explanation: 1. **Understanding Surface Tension:** - Surface tension is a physical property of liquids that arises from the cohesive forces between liquid molecules. These forces are stronger at the surface of the liquid because the molecules at the surface are not surrounded by similar molecules on all sides, leading to a net inward force. - This phenomenon causes the liquid to minimize its surface area, which is why droplets tend to form shapes that have the least surface area for a given volume—specifically, a sphere. 2. **Cohesive Forces:** - Cohesive forces are the attractive forces between molecules of the same substance. In the case of water, these forces are particularly strong due to hydrogen bonding. - When a droplet of water is placed on a solid surface, the cohesive forces between the water molecules pull the droplet into a shape that minimizes the surface area, resulting in a spherical shape. 3. **Why Option B is Correct:** - Option B accurately describes the role of surface tension in maintaining the spherical shape of droplets. The cohesive forces between the liquid molecules are what allow the droplet to resist deformation and maintain its shape. - The spherical shape is a direct consequence of surface tension, as it is the configuration that minimizes the energy associated with the surface area of the droplet. ### Why the Other Options are Incorrect: - **Option A: Surface tension causes the droplet to spread out uniformly across the surface.** - This statement is incorrect because surface tension actually works against spreading. If the adhesive forces (the forces between the liquid and the solid surface) are weaker than the cohesive forces (the forces between the liquid molecules), the droplet will not spread out but will instead form a more rounded shape. - **Option C: Surface tension has no significant effect on the shape of the droplet.** - This option is misleading. Surface tension is a critical factor in determining the shape of a droplet. Without surface tension, droplets would not maintain their shape and would instead flatten out or spread across the surface. - **Option D: Surface tension only influences the evaporation rate of the droplet.** - While surface tension can affect the evaporation rate (as it influences the surface area exposed to air), this statement is too narrow. Surface tension plays a much more significant role in determining the shape of the droplet, which is the primary focus of the question. ### Common Pitfalls: - Students often confuse the effects of cohesive and adhesive forces. Remember that cohesive forces lead to the droplet's shape, while adhesive forces determine how the droplet interacts with the surface. - It’s important to visualize the forces at play. Drawing a diagram of a droplet on a surface can help clarify how surface tension works to maintain the droplet's shape. ### Revision Summary: - Surface tension is caused by cohesive forces between liquid molecules, leading to a minimized surface area. - Droplets tend to form a spherical shape due to surface tension. - Adhesive forces can influence droplet behavior but do not dominate the shape when cohesive forces are strong. - Understanding the balance between cohesive and adhesive forces is key to grasping the behavior of liquid droplets on surfaces.