The correct option is **C. Density of the liquid**. ### Detailed Explanation **Surface Tension Overview:** 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. **Factors Affecting Surface Tension:** 1. **Temperature of the Liquid (Option A):** - As temperature increases, the kinetic energy of the molecules also increases. This increased motion allows molecules to overcome cohesive forces more easily, resulting in a decrease in surface tension. Therefore, temperature directly affects surface tension. 2. **Presence of Surfactants (Option B):** - Surfactants are substances that reduce surface tension when added to a liquid. They work by disrupting the cohesive forces between liquid molecules. For example, soap reduces the surface tension of water, allowing it to spread and wet surfaces more effectively. Thus, the presence of surfactants significantly affects surface tension. 3. **Density of the Liquid (Option C):** - Density is defined as mass per unit volume. While density can influence various properties of a liquid, it does not directly affect surface tension. Surface tension is primarily a result of intermolecular forces rather than the mass or volume of the liquid. Therefore, density is the factor that does NOT affect surface tension. 4. **Molecular Structure of the Liquid (Option D):** - The molecular structure, including the type of intermolecular forces (like hydrogen bonding, van der Waals forces, etc.), plays a crucial role in determining surface tension. For instance, water has a high surface tension due to strong hydrogen bonds between its molecules. Different molecular structures lead to different surface tension values. ### Why Other Options Are Incorrect or Weaker: - **Option A (Temperature of the liquid):** This option is incorrect because temperature has a clear and direct effect on surface tension, as explained above. - **Option B (Presence of surfactants):** This option is also incorrect because surfactants are specifically known to lower surface tension, making it a significant factor. - **Option D (Molecular structure of the liquid):** This option is incorrect as well, since the molecular structure is fundamental in determining the strength of intermolecular forces, which directly influence surface tension. ### Summary of Key Points: - Surface tension is influenced by temperature, surfactants, and molecular structure. - Increasing temperature decreases surface tension due to increased molecular motion. - Surfactants lower surface tension by disrupting cohesive forces. - Density does not directly affect surface tension, making it the correct answer to the question. ### Revision Summary: - Surface tension is affected by temperature, surfactants, and molecular structure. - Higher temperatures lead to lower surface tension. - Surfactants reduce surface tension significantly. - Density does not influence surface tension directly.