How does the polarity of water allow it to effectively dissolve ionic compounds?
Because water is a polar molecule (it has a permanent dipole), the water molecules surround the ion in an ordered fashion. The oxygen is slightly negative in water, while the hydrogens have a partial positive charge. So if you have a positive ion in water, there will be several water molecules around it all with their oxygens pointed at the positive ion. The partial negative charges on each oxygen all add up to cancel out the positive on the ion. That effectively cancels out the ion’s charge (a + combined with a – give zero), and so it is no longer attracted to another negative ion. (For a negative ion, it will be the hydrogens in water that face the ion.) How well a liquid can surround and stabilize an ion to “shield” its charge determines how well ionic compounds will dissolve.
