Question

Ionics compounds dissolve in water. But how can water which is a polar solvent break such a strong ionic bond such as that of NaCl? Polar molecules are weaker than ionic molecules

Answer 1

First, don't forget water molecules can exert very strong *repulsive* forces on the ions. When a water molecule moving at 200 m/s smashes into an ion, there is a very strong repulsive force exerted, like a hammer blow, that has nothing to do with the attractive ion-dipole or ion-ion forces.

Second, don't forget a whole group of water molecules can participate in prising just one ion out of the crystal structure.

Finally, the entire process is dominated by what in statistical mechanics we call "fluctuations" of the system away from its average properties. For example, an ion vibrating in place in the crystal momentarily vibrates much further away from equilibrium than it does on average, and in that instance the net attractive force from the water is more powerful than the attractive force from its neighboring ions. Off it goes. Or one water happens to have an unusually high velocity and hits a juncture between neighboring ions at just the right angle to split them apart, like a wedge driving between the two halves of a log.

These fluctuations go on all the time, as long as the temperature is above zero. And, in fact, ions will be breaking off from the crystal all the time, under any conditions at all, even those under which the ionic compound does not dissolve (e.g. in nonpolar solvent). However, it is also the case that all the time dissolved ions crash into the crystal and re-integrate, reforming a tiny bit of the crystal. There is, in short, a dynamic equilibrium, for example:

NaCl(s) <-> Na+(aq) + Cl-(aq)

Whether on balance the forward or reverse reaction dominates *does* depend on things like the free energy change, which depends on the average attractive forces, and the temperature. So when it comes to deciding whether the tiny bits of crystal that are breaking off all the time *stay* broken off, or whether the handful of ions in solution *stay* in solution, or rapidly rejoin the crystal, it does matter what the attractive forces are on average.

If your question is why, since the ion-dipole interactions of the dissolved ions are weaker than the ion-ion interactions of the same ions in the crystal, does the crystal dissolve at all, the answer is entropy: when the ions dissolve their entropy rises steeply, and this compensates for the increase in enthalpy.

A mechanistic explanation for that is that when the ions dissolve, the solution is so large that nearly all of them "get lost" wandering around the solution and never wander back to crash into the crystal again and re-integrate, before the crystal dissolves fully.

Answer 2

water molecules are highly polar due to the extreme difference of electronegativity between H atoms and O atom-the electron from the H atom is closer to the O atom, so the H sides are slightly positive and the O side is slightly negative

since water molecules are polar, ionic compounds, can be separated by water molecules because of the +ve -ve attraction between ions and the water molecules