The correct option is **A. The temperature of the liquid**. ### Detailed Explanation **Surface Tension Defined:** Surface tension is a physical property of liquids that describes the elastic-like force existing at the surface of a liquid. It arises from the cohesive forces between liquid molecules. Molecules at the surface experience a net inward force due to the attraction from molecules below them, leading to a minimized surface area. **Why Temperature Affects Surface Tension:** 1. **Molecular Motion:** As the temperature of a liquid increases, the kinetic energy of its molecules also increases. This increased motion allows molecules to overcome intermolecular forces more easily. 2. **Cohesive Forces:** At higher temperatures, the cohesive forces between molecules weaken. This means that the molecules at the surface are less tightly bound to each other, resulting in a decrease in surface tension. 3. **Empirical Observations:** Experimental data shows that for most liquids, surface tension decreases with an increase in temperature. For example, the surface tension of water decreases from about 72.8 mN/m at 20°C to about 58.9 mN/m at 100°C. ### Why the Other Options Are Incorrect or Weaker **B. The density of the liquid:** - **Explanation:** While density can influence some properties of liquids, it does not directly affect surface tension. Surface tension is more about the intermolecular forces at the surface rather than how closely packed the molecules are. For example, both water and mercury have high densities, but their surface tensions are vastly different due to the nature of their intermolecular forces. **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 fluid. While viscosity can be influenced by temperature and can affect how surface tension manifests in dynamic situations (like in bubbles or droplets), it does not primarily determine the surface tension itself. For instance, honey has a high viscosity but does not have a high surface tension compared to water. **D. The molecular weight of the liquid:** - **Explanation:** Molecular weight can influence the properties of a liquid, but it is not a primary factor affecting surface tension. Surface tension is more closely related to the type of intermolecular forces (like hydrogen bonding in water) rather than just the weight of the molecules. For example, both water (molecular weight 18 g/mol) and ethanol (molecular weight 46 g/mol) have different surface tensions due to their molecular interactions, not just their weights. ### Summary of Key Points - Surface tension is primarily affected by the temperature of the liquid; as temperature increases, surface tension decreases. - Increased molecular motion at higher temperatures weakens cohesive forces, leading to lower surface tension. - Density, viscosity, and molecular weight do not primarily determine surface tension, although they can influence other properties of liquids. - Understanding the relationship between temperature and surface tension is crucial for applications in physics and engineering, such as in fluid dynamics and material science. This comprehensive understanding of surface tension and its influencing factors will aid in your preparation for professional exams in physics.