The correct option is **D. surface tension**. ### Explanation of the Correct Answer **Surface Tension** is a physical property of liquids that describes the elastic tendency of a fluid surface. It is caused by the cohesive forces between liquid molecules. In the case of water, these cohesive forces are particularly strong due to hydrogen bonding, which causes water molecules to stick together. When water is in contact with a surface, such as the fabric of an umbrella, the surface tension creates a sort of "skin" on the water's surface. This means that the water droplets tend to maintain a spherical shape and resist spreading out. When the umbrella is open, the water droplets do not fall through the silk material because the surface tension holds them together, preventing them from breaking apart and allowing them to pass through the fabric. When you touch the inside of the umbrella, you disrupt the surface tension. Your finger introduces a new surface for the water to interact with, which can break the cohesive forces holding the water droplets together. This allows the water to spread out and eventually drip through the umbrella. ### Why the Other Options Are Incorrect **A. Capillarity**: - Capillarity (or capillary action) refers to the ability of a liquid to flow in narrow spaces without the assistance of external forces. It is primarily observed in small tubes or porous materials. While capillarity is related to surface tension, it does not explain why water does not drop through the umbrella unless touched. The umbrella's fabric does not create a narrow space for capillary action to occur; instead, it is the surface tension that keeps the water droplets intact. **B. Osmotic Pressure**: - Osmotic pressure is the pressure required to prevent the flow of a solvent into a solution through a semipermeable membrane. It is a concept primarily used in biology and chemistry to describe the movement of water across cell membranes. Osmotic pressure is not relevant to the situation with the umbrella and water droplets, as it does not pertain to the behavior of water on a fabric surface. **C. Viscosity**: - Viscosity is a measure of a fluid's resistance to flow. While it affects how easily a liquid can move, it does not explain the phenomenon of water droplets remaining on the umbrella. Viscosity would be more relevant if we were discussing how quickly water flows or spreads, but it does not account for the cohesive forces that keep the droplets from falling through the fabric. ### Summary of Key Concepts - **Surface Tension**: The cohesive forces between liquid molecules create a "skin" on the surface, preventing water from passing through the umbrella. - **Capillarity**: Not applicable in this scenario as it involves narrow spaces, which are not present in the umbrella's fabric. - **Osmotic Pressure**: Irrelevant to the behavior of water droplets on the umbrella. - **Viscosity**: Relates to the flow of liquids but does not explain the retention of water droplets on the umbrella. ### Revision Summary - Water droplets remain on the umbrella due to **surface tension**, which holds them together. - Touching the inside of the umbrella disrupts surface tension, allowing water to drip through. - Capillarity, osmotic pressure, and viscosity do not explain this specific phenomenon. - Understanding surface tension is crucial for grasping how liquids behave on different surfaces.