Q.Why naphthalene dissolves in benzene but not in water?

Step 1. Naphthalene (C10H8) is a nonpolar aromatic hydrocarbon; the only intermolecular forces holding solid naphthalene together, and the only forces it can form with a solvent, are weak van der Waals (London dispersion) forces.

Step 2. Benzene is also nonpolar, held together by the same kind of weak van der Waals forces. When naphthalene dissolves in benzene, the naphthalene-benzene interactions, benzene-benzene interactions, and naphthalene-naphthalene interactions are all of similarly weak strength -- so there's no large energy cost to breaking up the pure substances and mixing them, and 'like dissolves like' applies directly.

Step 3. Water, by contrast, is strongly polar and hydrogen-bonded -- water molecules attract each other very strongly via hydrogen bonds. For naphthalene to dissolve in water, these strong water-water hydrogen bonds would have to be broken to make room for nonpolar naphthalene molecules, but naphthalene can offer only weak van der Waals attraction in return -- nowhere near enough to compensate energetically for the strong hydrogen bonds being disrupted.

Step 4. Because the naphthalene-water interaction is so much weaker than the water-water interaction it would have to replace, naphthalene stays essentially insoluble in water, even though it is easily soluble in a nonpolar solvent like benzene.

Apply the 'like dissolves like' rule by comparing the type and strength of intermolecular forces in naphthalene, benzene, and water.

• Assuming any organic compound is automatically 'nonpolar' -- polarity depends on the specific molecular structure, not just on being 'organic'.
• Forgetting that the barrier to dissolving in water is not naphthalene's own weakness, but water's UNUSUALLY strong self-attraction (hydrogen bonding) that has to be overcome.