Surface tension decreases with increasing temperature because the increased kinetic energy of the molecules weakens the intermolecular forces responsible for surface tension.
Here's a more detailed explanation:
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Intermolecular Forces and Surface Tension: Surface tension arises from cohesive forces between liquid molecules. Molecules in the bulk of the liquid experience these forces equally from all directions. However, molecules at the surface experience a net inward force, pulling them into the liquid and minimizing the surface area. This inward force is what creates surface tension.
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Temperature and Kinetic Energy: Temperature is a measure of the average kinetic energy of the molecules. As temperature increases, the molecules move faster and have more kinetic energy.
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Weakening of Intermolecular Forces: The increased kinetic energy weakens the effect of the intermolecular forces. The molecules are more likely to overcome the attractive forces and move away from each other. This reduces the net inward force on the surface molecules, leading to a decrease in surface tension. Think of it like this: the "stickiness" of the molecules reduces as they become more energetic.
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Example: Consider water. At room temperature, water has a significant surface tension. However, when water is heated, its surface tension decreases, making it easier for it to spread out. This is why hot water is often used for cleaning; its lower surface tension allows it to penetrate into smaller spaces and dissolve dirt more effectively.
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Summary Table:
Factor Effect on Surface Tension Explanation Increasing Temperature Decreases Increased kinetic energy weakens intermolecular forces, reducing net inward force.
In essence, as temperature rises, the liquid molecules become more agitated and less attracted to one another, leading to a reduction in the force required to break the surface and thus a lower surface tension.
