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What is the real cause behind the cohesive and adhesive forces?

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en.wikipedia.org/wiki/Van_der_Waals_force the Van der Waals force(s) –  anna v Mar 17 '13 at 14:02
    
@annav what is london dispersion force? –  newera Mar 17 '13 at 23:31
    
In the link above there exists a link for your question. The highlighted words are links: en.wikipedia.org/wiki/London_dispersion_force , it "is a weak intermolecular force arising from quantum induced instantaneous polarization multipoles in molecules. They can therefore act between molecules without permanent multipole moments." –  anna v Mar 18 '13 at 4:42
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Most molecules have a dipole moment, and are called "polar". This means that the electronsare not uniformly distributed throughout; rather there are some areas with a net positive charge (due to lack of electrons), and some with a net negative charge (due to abundance of electrons). Even water is polar; the electrons in the O-H bond are closer to the oxygen molecule, giving oxygen a slightly negative charge.

Now, the force of attraction between dipoles is what makes upVan Der Waals forces. Cohesion and adhesion are places where this force crops up.

Note that not all molecules are polar. Apolar ones can acquire a temporary "induced" charge like so:

enter image description here

This leads to dipole-induced dipole attraction. There also are induced dipole-indiced dipole interactions, when two apolar molecules form extremely temporary dipoles, and line up.

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