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It is known that molecules at the surface are more strongly attached to each other (more attraction less repulsion) than those within the bulk (attraction and repulsion are balanced). This is the molecular description of the surface tension and its net direction i.e., parallel to the water surface. Briefly, considering energy (total energy cost) - the cost of excluding a surface molecule is less, than to break a molecule from bulk and evaporate it from the surface!

Now, considering evaporation, why does evaporation occur through molecules of the surface (stronger attachment than those of the bulk) and not through molecules from the bulk (like assumed in boiling)?

Martin
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mike
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1 Answers1

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It is known that molecules at the surface are strongly attached to each other (more attraction less repulsion) than those within the bulk attraction and repulsion are balanced).

I don't think that is an accurate description. Molecules in the bulk are maximally surrounded by neighbors and experience the most attractive force. The forces on a bulk molecule are balanced directionally. Molecules at the surface have fewer neighbors and experience less attractive forces. The forces on a surface molecule are not balanced, there is net attraction in the inward direction with respect to the fluid.

why evaporation occurs towards molecules of the surface (the stronger attachment than those of the bulk)

Again, the surface molecules experience less intermolecular attractive forces, not more. This is why liquids are in the lowest energy state when surface area is minimized. Droplets are spherical absent external forces.

DavePhD
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