Higher temperatures result in lower surface tensions. This is because temperature is a measure of kinetic energy in a substance, or how fast the molecules are moving. If liquid molecules are moving quickly, they have less time to interact with each other and thus have less attractive forces between them, reducing the surface tension. Adding substances to a liquid can have different effects on surface tension depending on the liquid and substance being added. Surfactants are molecules that are designed specifically to reduce surface tension. However, dissolving salts in water typically increases surface tension, and dissolving sugar in water has no effect.
Van der Walls forces influence surface tension, as well as many other properties, such as boiling point or melting point. In general, if there are stronger intermolecular forces, the molecules tend to hold together more strongly because they are more attracted to each other. Because of this attraction, the surface tension and boiling point will tend to be higher. Surface tension is a result of a substance holding together with itself, so you would imagine that a substance with strong intermolecular forces (like water) would have a high surface tension. These molecules at the surface form a layer and you need a certain amount of energy to overcome and break the surface layer. When you heat something up, it adds energy, and that energy can overcome the surface tension.You can also add things to the substance to interfere or disrupt its intermolecular forces. For example, water has a high surface tension because it is strongly attracted to itself. However, if you add alcohol, instead of water interacting only with other water molecules, it now interacts (less strongly) with alcohol, and the surface tension of the mixture will be lower.
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