**Correct Option: B. The temperature of the liquid** ### Detailed Explanation **Understanding Surface Tension:** Surface tension is a physical property of liquids that describes the elastic-like force existing at the surface of a liquid. It arises due to the cohesive forces between liquid molecules. Molecules at the surface experience a net inward force because they are attracted more strongly to the molecules beside and below them than to the air above. This creates a "skin" on the surface of the liquid, which can affect how objects interact with it. **Why Temperature Affects Surface Tension:** 1. **Molecular Motion:** As the temperature of a liquid increases, the kinetic energy of its molecules also increases. This means that the molecules move faster and are more energetic. 2. **Cohesive Forces:** At higher temperatures, the increased motion of the molecules leads to a decrease in the cohesive forces between them. This is because the faster-moving molecules are less likely to be held together tightly, which reduces the strength of the surface tension. 3. **Empirical Observations:** Experiments show that for most liquids, as the temperature rises, the surface tension decreases. For example, water has a surface tension of about 72.8 mN/m at 20°C, which decreases to about 58.9 mN/m at 100°C. ### Why the Other Options Are Incorrect **A. The mass of the liquid** - **Explanation:** The mass of the liquid does not directly affect surface tension. Surface tension is a property that depends on the intermolecular forces and the arrangement of molecules at the surface, not on the total mass of the liquid. Two liquids with the same mass can have very different surface tensions depending on their molecular structure and interactions. **C. The viscosity of the liquid** - **Explanation:** Viscosity is a measure of a fluid's resistance to flow and is related to the internal friction between layers of the liquid. While viscosity can be influenced by temperature (and thus indirectly related to surface tension), it is not a primary factor affecting surface tension itself. A liquid can have high viscosity and still have low surface tension, or vice versa. **D. The density of the liquid** - **Explanation:** Density is the mass per unit volume of a substance. While density can influence how a liquid behaves in certain contexts (like buoyancy), it does not directly determine surface tension. Different liquids can have the same density but vastly different surface tensions due to differences in molecular interactions. ### Summary of Key Points - Surface tension is primarily affected by the temperature of the liquid; as temperature increases, surface tension generally decreases. - The mass of the liquid, viscosity, and density do not directly influence surface tension. - Understanding the molecular interactions and kinetic energy of molecules is crucial in grasping how temperature affects surface tension. ### Revision Summary - Surface tension is the force at the surface of a liquid due to cohesive forces between molecules. - Temperature is the primary factor affecting surface tension; higher temperatures decrease surface tension. - Mass, viscosity, and density do not directly affect surface tension. - Always consider molecular interactions when studying properties like surface tension.