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For instance, in Russian there is an adjective "oily", which means a liquid of high viscosity. Is this assertion even true in general or statistically?

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    $\begingroup$ Water and benzene, as an example of polar and non-polar solvent, do not differ much in their visosities reported here. $\endgroup$
    – Buttonwood
    Jun 27 at 17:01
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    $\begingroup$ Non-polar pentane is much less viscous than water. So the assertion is clearly not true in general. It might still be true as a very vague and imprecise rule of thumb, much like "all liquids have density 1". $\endgroup$ Jun 27 at 17:06
  • $\begingroup$ Having long chain or ring molecules that get tangled up easily sounds like a good way to increase viscosity, compared with a small molecules like water. If we compare liquids whose molecules are more or less similar in size and shape we probably find that polar ones are more viscous. $\endgroup$ Jun 27 at 18:41
  • $\begingroup$ @OscarLanzi but is there a single highly-viscous polar solvent? $\endgroup$
    – Anixx
    Jun 27 at 19:43
  • $\begingroup$ When I first read this I thought you had gotten things the wrong way around. I usually associate higher viscosity with H-bonding liquids like water. Consider just pentane and hexane in eg en.wikipedia.org/wiki/List_of_viscosities. Non-volatile oils are not volatile because of their high b.p. which is due to their high MW, as a result of which they are also viscous. $\endgroup$
    – Buck Thorn
    Jun 27 at 20:15
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Viscosity depends on the intermolecular forces between molecules. Typically, the more viscous a substance or liquid is, the stronger its intermolecular forces are. In polar solvents, there are dipole-dipole interactions while in non-polar solvents, there are London Dispersion Forces. Sometimes, when individual molecules are large, LDF can be stronger than dipole-dipole interactions in some polar solvents. The point is that having stronger intermolecular forces will lead to higher viscosity, and being polar doesn't mean it will have stronger IMF than non-polar substances.

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