|University Federico II
of Naples, Italy
Water molecule is polar
The molecule of water, H2O, is not linear, and the centre of the positive charges does not coincide with the centre of the negative charges. Hence, it is a polar molecule (a dipole): that has effects on the interactions between charged groups, and between polar, but uncharged, groups.
The hydrated charged ion Na+
Because of its polarity, water is a good solvent for many ionic compounds. For instance, the familiar salt used to cook, sodium chloride, NaCl, is very soluble in water, owing to its dissociation in two charged species, the two ions Na+ and Cl-, stabilized by the strong electrostatic interactions between each ion and the H2O dipole.
Structure of glucose
Many uncharged substances dissolve readily in water because they bear polar functional groups, which form favourable interactions with water molecules. Alcohols, amines, amides, ketones, etc. are included in this kind of substances. Ionic and polar compounds are called hydrophilic (water loving), just for their favourable interactions with water molecules. For instance, the molecule of a sugar, as glucose, bearing many OH groups which interact well with water, breaks water-water hydrogen bonds forming glucose-water hydrogen bonds.
These bonds are stronger than the interactions which would cause two glucose molecules to attract each other. As a consequence, glucose is very soluble in water, and the amount that can be dissolved in water is so large that the solution becomes, as said above, "syrupy".