The correct option is **B. Surface tension allows liquids to form droplets due to cohesive forces exceeding adhesive forces.** ### Detailed Explanation: **Understanding Surface Tension:** Surface tension is a physical property of liquids that arises from the cohesive forces between liquid molecules. These forces are due to intermolecular attractions, such as hydrogen bonding in water. At the surface of a liquid, molecules experience a net inward force because they are surrounded by other liquid molecules on the sides and below, but not above. This creates a "skin-like" effect on the surface of the liquid. **Cohesive vs. Adhesive Forces:** - **Cohesive Forces:** These are the attractive forces between molecules of the same substance. In the case of water, the hydrogen bonds between water molecules create strong cohesive forces. - **Adhesive Forces:** These are the attractive forces between molecules of different substances, such as water molecules and molecules of a solid surface (like glass or wood). When a liquid comes into contact with a solid surface, the behavior of the liquid depends on the relative strengths of these forces: - If cohesive forces (within the liquid) are stronger than adhesive forces (between the liquid and the solid), the liquid will tend to minimize its contact with the solid surface, leading to the formation of droplets. - Conversely, if adhesive forces are stronger, the liquid will spread out more uniformly over the surface. **Why Option B is Correct:** Option B correctly states that surface tension allows liquids to form droplets due to cohesive forces exceeding adhesive forces. This is particularly evident with water on a non-wettable surface (like wax paper), where water beads up into droplets rather than spreading out. The cohesive forces pull the water molecules together, forming a shape that minimizes surface area, which is characteristic of droplets. ### Why the Other Options are Incorrect: **A. Surface tension causes liquids to spread uniformly over solid surfaces.** - This statement is incorrect because it overlooks the balance between cohesive and adhesive forces. If cohesive forces are stronger, the liquid will not spread uniformly; instead, it will form droplets. This option fails to recognize the critical role of surface tension in determining the shape of the liquid. **C. Surface tension has no significant effect on the shape of liquids in contact with solids.** - This option is misleading. Surface tension significantly affects the shape of liquids in contact with solids. It is the reason why liquids can form droplets or spread out, depending on the interaction with the solid surface. Saying it has "no significant effect" is incorrect. **D. Surface tension only affects the movement of gases and not liquids.** - This statement is false. Surface tension is a property that specifically pertains to liquids. It does not apply to gases in the same way because gases do not have a defined surface tension; they expand to fill their containers. This option misrepresents the fundamental nature of surface tension. ### Summary of Key Points: - Surface tension is caused by cohesive forces between liquid molecules, leading to a "skin-like" effect on the surface. - The behavior of a liquid on a solid surface depends on the balance of cohesive and adhesive forces. - When cohesive forces exceed adhesive forces, liquids form droplets rather than spreading out. - Understanding surface tension is crucial for predicting how liquids behave in contact with different materials. This thorough understanding of surface tension will help you grasp how liquids interact with solids, which is essential for various applications in physics and engineering.