### Correct Option: A. Decreases #### Detailed Explanation: 1. **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 to other liquid molecules below them, leading to a minimized surface area. 2. **Effect of Temperature on 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 collide with each other more vigorously. The increased motion disrupts the cohesive forces that hold the molecules together at the surface. 3. **Decrease in Surface Tension**: - With increased molecular motion, the cohesive forces between the molecules at the surface become weaker. As a result, the surface tension decreases. This is because the molecules at the surface are less tightly bound to each other, allowing the surface to stretch more easily. 4. **Experimental Evidence**: - Numerous experiments have shown that as the temperature of a liquid (like water) increases, the surface tension decreases. For example, the surface tension of water at 0°C is about 72.8 mN/m, while at 100°C, it drops to about 58.9 mN/m. #### Why Other Options Are Incorrect: - **Option B: Increases**: - This option is incorrect because increasing temperature leads to increased molecular motion, which weakens the cohesive forces at the surface, thereby reducing surface tension rather than increasing it. - **Option C: Remain Constant**: - This option is also incorrect. Surface tension does not remain constant with temperature changes. The relationship between temperature and surface tension is well-documented, showing a clear trend of decreasing surface tension with increasing temperature. - **Option D: Increases then decreases**: - This option suggests a two-phase behavior, which is not observed in typical liquids. While some complex fluids may exhibit non-linear behavior under specific conditions, for most common liquids, surface tension consistently decreases with increasing temperature without an initial increase. #### Formulas and Common Pitfalls: - **Formula for Surface Tension**: - Surface tension (γ) can be defined as the force (F) per unit length (L) acting along the surface: \[ \gamma = \frac{F}{L} \] - However, this formula is more about the measurement of surface tension rather than its dependence on temperature. - **Common Pitfalls**: - Students often confuse surface tension with viscosity. While both are properties of liquids, viscosity relates to a liquid's resistance to flow, while surface tension relates to the cohesive forces at the surface. - Another common mistake is assuming that all liquids behave the same way with temperature changes. While the trend of decreasing surface tension with increasing temperature is common, the exact values and rates of change can vary between different liquids. ### Revision Summary: - Surface tension decreases as the temperature of a liquid increases due to increased molecular motion. - Increased kinetic energy weakens the cohesive forces at the surface, leading to lower surface tension. - Common liquids like water show a clear decrease in surface tension with temperature rise. - Remember the distinction between surface tension and viscosity, as they are different properties.