Question

What are the three reason why substance dissolve?

Answer 1

The solvent weakens the ionic or the covalent bond between the constituents of the solid, separates them, and then carries them away from each other.
Look up ionising/dissociating/solvating solvent.

Look at second paragraph (Solvents and intermolecular interactions) of the Wikipedia page:
http://en.wikipedia.org/wiki/Solvation

Answer 2

Generally speaking, dissolution may in principle occur for enthalpic reasons, because the created solute-solvent interactions are stronger than the lost solute-solute and solvent-solvent interactions, but this is rare. Much more commonly, solution is entropically drive, meaning it occurs because of the tremendous increase in entropy when a solute is dispersed in the solvent. Finally, in some cases a chemical reaction may occur between the solute and solvent, and the results of the reaction are more soluble than the original solute.

Answer 3

1) absorption 2) concentration gradient 3) osmosis or diffusion