A solvent is a liquid substance capable of dissolving other substances. Why is it that water cannot dissolve sand...or vegetable oil...but dissolves salt and sugar?
Remember our discussion of ionic molecules?
An ionic molecule is a molecule that has atoms in it that have either lost or gained electrons, changing the electrical charges in that atom.
Table salt is made up of molecules which contain sodium ions and chloride ions. The sodium ions results when a sodium atom loses an electron and as a result are positively charged. The chloride ion results when a chloride atom gains an electron, and are therefore negatively charged. As a result the ions come together to make a molecule. When an ionic compound like salt is put in water, the electrical charges in the water molecules attract the electrical charges in the ions, pulling the ions away from each other. Eventually each ion is surrounded by water molecules and pulled so far away from the other ions that the substance is no longer visible in the solution. Although still there, the ions are so far removed from each other that they exist on their own. Since the ions are too small to see, the substance seems to disappear.
So water dissolves substances made up of either polar molecules or ionic molecules. If a molecule, on the other hand, is non-polar, such as sand or vegetable oil, water will not be attracted to it since it does not have a net electrical charge.
source: Exploring Creation with Physical Science, Jay Wile