The correct option is **B. Surface tension causes the droplet to form a perfect sphere, minimizing its surface area.** ### Detailed Explanation: 1. **Understanding Surface Tension:** - Surface tension is a physical property of liquids that arises from the cohesive forces between liquid molecules. Molecules at the surface of a liquid experience a net inward force because they are attracted to other liquid molecules, which creates a "skin" effect on the surface. - This phenomenon causes the liquid to minimize its surface area, as a smaller surface area means fewer molecules are exposed to the air, thus reducing the energy associated with the surface. 2. **Shape of a Liquid Droplet:** - When a droplet of liquid is placed on a solid surface, surface tension acts to pull the molecules at the surface inward. The shape that minimizes the surface area for a given volume is a sphere. - Therefore, in the absence of other forces (like gravity or adhesion to the surface), a droplet will naturally take on a spherical shape due to surface tension. 3. **Role of Gravity and Adhesion:** - While gravity does play a role in the overall shape of a droplet, especially when it is large enough to deform under its weight, surface tension is the dominant force that dictates the initial shape of small droplets. - If the droplet is small enough, the effect of gravity is negligible compared to the cohesive forces due to surface tension, allowing the droplet to maintain a nearly spherical shape. 4. **Why Other Options Are Incorrect:** - **Option A: Surface tension causes the droplet to spread out completely and form a thin layer.** - This statement is incorrect because surface tension actually works against spreading. If the adhesive forces between the liquid and the solid surface are weaker than the cohesive forces within the liquid, the droplet will not spread out completely. Instead, it will maintain a shape that minimizes surface area, which is spherical. - **Option C: Surface tension has no effect on the shape of the droplet; it is solely determined by gravity.** - This is misleading. While gravity does influence the droplet's shape, especially for larger droplets, surface tension is the primary factor for small droplets. In fact, for small droplets, surface tension is the dominant force, and gravity's effect is minimal. - **Option D: Surface tension causes the droplet to evaporate faster than it would in a flat layer.** - This statement is not accurate in the context of the droplet's shape. Surface tension does not directly affect the rate of evaporation in this manner. Evaporation is influenced by factors such as temperature, surface area, and humidity, but surface tension itself does not cause droplets to evaporate faster than a flat layer. ### Summary of Key Points: - Surface tension causes liquid droplets to minimize their surface area, leading to a spherical shape. - The spherical shape is a result of cohesive forces among liquid molecules, which dominate over gravitational forces in small droplets. - Gravity can influence larger droplets, but surface tension is the primary factor for small droplets. - Other options incorrectly describe the effects of surface tension or misinterpret the role of gravity. This understanding of surface tension and droplet behavior is crucial for various applications in physics, chemistry, and engineering, including fluid dynamics and material science.