How Dissolving Happens

A water molecule, ${\text{H}}_2\text{O}$, is bent in shape with oxygen pulling electron density toward itself (oxygen is more electronegative than hydrogen). This gives water a partial negative charge (${\delta }^{-}$) near the oxygen and partial positive charges (${\delta }^{+}$) near the two hydrogens — water is a polar molecule, acting like a tiny magnet with a negative end and a positive end.

An ionic solid like $\text{NaCl}$ is a rigid lattice of alternating ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ ions held together by strong electrostatic (ionic) attractions. When the crystal is dropped into water, polar water molecules cluster around the surface: their negative (oxygen) ends point toward ${\text{Na}}^{+}$ ions, and their positive (hydrogen) ends point toward ${\text{Cl}}^{-}$ ions. Enough water molecules pulling from enough directions can overcome the lattice's internal attraction and pry individual ions away from the crystal, surrounding each free ion in a shell of water molecules. This surrounding process is called hydration (or more generally, solvation). The ions don't get created by this process — they already existed in the solid lattice; water simply separates and surrounds them. This separation of an ionic compound into its pre-existing ions is called dissociation.

Molecular compounds have no ions to begin with — they're made of individual covalent molecules. If the molecule is polar (like sugar or ethanol, which have $\text{-OH}$ groups), water molecules can still cluster around it and interact through hydrogen bonding and dipole-dipole attractions, pulling individual molecules away from each other and into solution — but the molecule itself stays intact as a whole, uncharged unit. No ions are produced, which is why molecular solutes like sugar are non-electrolytes. Polarity is a matter of degree, and it shows up clearly in the alcohol family. Methanol and ethanol, with short carbon chains, are miscible with water in all proportions. As the non-polar hydrocarbon chain gets longer it takes over: butanol (four carbons) dissolves only partly, about 7 g per 100 mL of water, and octanol (eight carbons) barely dissolves at all. The –OH end still hydrogen-bonds with water, but the long oily tail cannot, and it disrupts more of water's hydrogen-bond network than the single –OH group can make up for.

1. Separate the solute particles — ions or molecules in the solid must be pulled away from each other. This absorbs energy, because attractions are being overcome.
2. Separate the solvent particles — water molecules must be pushed apart to make room, breaking some of their hydrogen bonds. This also absorbs energy.
3. Form solute–solvent attractions — the freed particles are hydrated through ion–dipole attractions or hydrogen bonds. This releases energy.
4. Compare the two. If more energy is released in step 3 than absorbed in steps 1 and 2, the solution warms up (exothermic dissolving: ${\text{CaCl}}_2$ in hot packs, NaOH pellets). If more is absorbed than released, it cools (endothermic dissolving: ${\text{NH}}_4{\text{NO}}_3$ in instant cold packs). Some endothermic dissolving still happens on its own because particles naturally tend to spread out and mix, but if step 3 is far too weak — as with oil in water — the solute stays undissolved.

This rule of thumb summarizes when dissolving will actually happen. Polar solvents (like water) are good at dissolving polar and ionic solutes, because the attractive forces between solvent and solute (ion-dipole or dipole-dipole/hydrogen bonding) are strong enough to overcome the solute's internal attractions. Non-polar solvents (like oil or hexane) are good at dissolving non-polar solutes, through weak dispersion forces, but cannot generate strong enough attractions to pull apart a polar or ionic solute. This is exactly why oil and water don't mix: water is polar, oil is non-polar, and neither can out-compete the other's internal attractions. The rule explains a lot of everyday chemistry. Soap works because each soap particle has an ionic 'head' that dissolves in water and a long non-polar 'tail' that dissolves in grease, so it can carry oily dirt away into the rinse water. Vitamins A, D, E and K are largely non-polar, so they dissolve in body fat and can build up over time, while vitamins B and C are polar and water-soluble, so excess amounts are flushed out. Dry cleaners use non-polar solvents to lift grease stains that water alone cannot touch.