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Both are made up of small particles. Perhaps the particles in a powder are larger than those of a liquid (multiple molecules vs. a single molecule?); but it does not seem generally possible to grind a powder into a liquid.

Is it something about how the particles interact?

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  • $\begingroup$ powdering a substance means making fine tiny crystallites from a solid material. Liquefying the same means expanding it. $\endgroup$ – UKH Jan 28 '17 at 4:36
  • $\begingroup$ To have liquid-like behavior, you need an attractive interaction between the particles of your fluid (in this case, a fine powder). $\endgroup$ – valerio Mar 11 '18 at 0:00
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    $\begingroup$ I think this question is very hard because powders can show liquid-like behavior, eg. dry dirt during an earthquake. I saw a comment about it in Greenside and Cross "Pattern Formation" mentioning that understanding how hydrodynamics can emerge in a granular situation is an open problem. $\endgroup$ – Ryan Thorngren Apr 10 '18 at 23:23
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A solid is characterised by the fact that the atoms within it do not have sufficient energy to break the bonds between the atoms.
So an atom in a solid always has the same nearest neighbours. Raising the temperature will increase the kinetic energy of the atoms and eventually some of the bonds between atoms can be broken so that the atoms can move relative to one another.

If you ground your solid particles at constant temperature and exposed "clean" surfaces on the particles then the atoms from different particles might form bonds thus making a bigger particle again.
However you will not form a liquid unless you raised the temperature and then you probably would not have to do any grinding.
Usually particles do not join together because their surfaces are contaminated.
However materials with ultra clean surfaces can be cold welded.

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