What is special about water as a solvent?

1 Answer
Jan 28, 2016

The bonds in water are POLAR. The result is that water has positive and negative ends, potentially capable of solvating discrete ions.


POLARITY is a result of charge separation. Because oxygen is more electronegative than hydrogen, it polarizes electron density towards itself such that oxygen acquires a partial negative charge, and the hydrogens each acquire a partial positive charge.

We can represent this bond polarity, this charge separation, by #""^(delta+)H-""^(delta-)O-H^(delta+)#. Because of this polarity, the water molecule can interact with ionic solids, and potentially solvate positive and negative ions, if the electotstatic interaction between anion and cation is not too strong.

In aqueous solution we would represent an ionic compound, say #NaCl#, as #NaCl(aq)#, when the actual microscopic picture would be #[Na(OH_2)_6]^+# and #[Cl(H_2O)_(4-6)]^-#. This is what we mean by the aquated complex, i.e. #Na(aq)^+#. In each case the individual ion is solvated by water molecules, that are bound to the ion by ion-dipole interactions.

In aggregate these ion-dipole interactions are strong enough to compensate for the breaking of the ion-ion interaction in solid sodium chloride. (In fact, the dissolution of sodium chloride in water is very slightly endothermic; the formation of new ion-dipole bonds is ALMOST enough to compensate for the breaking of the ion-ion interactions present in the solid.) Even a solvent such as ethanol (which does contain an hydroxyl group) is insufficiently polar to solvate discrete sodium and chloride ions.

By contrast organic solvents, such as hexanes or octanes or ethers (or oils) are incapable of solvating either positive or negative ions, because there is no similar degree of polarity for these solvents. There are no new bonds formed that can compensate for the breaking of ion-ion bonds; hence solubility of these ions in these media is untenable.