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My physics textbook says that when partially soluble substances are added to a liquid, it's surface tension decreases while on adding completely soluble substance, surface tension increases. There is no further discussion. I am not able to figure out what actually might be happening to give the observed results .

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Partially soluble substances will populate interfaces, and thus decrease by some ratio (depending among other things on adsorbing/desorbing rates and on concentration) the variations of surface energy associated with the variation of interfacial area.

This will not be the case with completely soluble substances, which will remain in the bulk and whose effect can hence be only to increase the surface tension (although they may also have a neutral effect on it).

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