Water as a Solvent

Water as a Solvent

The partial charge that develops across the water molecule helps make it an excellent solvent. Water dissolves many substances by surrounding charged particles and "pulling" them into solution. For example, common table salt, sodium chloride, is an ionic substance that contains alternating sodium and chlorine ions.
NaCl-crystal - Sodium chloride contains Na+ and Cl- ions.

Sodium chloride contains Na+ and Cl- ions.

When table salt is added to water, the partial charges on the water molecule are attracted to the Na+ and Cl- ions. The water molecules work their way into the crystal structure and between the individual ions, surrounding them and slowly dissolving the salt. The water molecules will actually line up differently depending on which ions are being pulled into solution. The negative oxygen ends of water molecules will surround the positive sodium ions; the positive hydrogen ends will surround the negative chlorine ions.
NaCl-dissolve - Table Salt Dissolving in Water

Table Salt Dissolving in Water

In a similar fashion, any substance that carries a net electrical charge, including both ionic compounds and polar covalent molecules (those that have a dipole), can dissolve in water. This idea also explains why some substances do not dissolve in water. Oil, for example, is a nonpolar molecule. Because there is no net electrical charge across an oil molecule, it is not attracted to water molecules and therefore does not dissolve in water.

From: http://www.visionlearning.com/library/module_viewer.php?c3=&mid=57&l=