The process of a substance dissolving into an aqueous solution (water) can be viewed as a reaction of the substance (solute) with the water molecules (solvent) to form a solution. Water is a "polar" molecule. The angle between the H-0-H is 104.5 degrees. Because oxygen is much more electronegative (attracts electrons more), the electrons tend to stay more around the oxygen atom than the hydrogen atoms, even though the water molecule is overall electrically neutral. The shape of the molecule results in each water molecule having a partially positive end (oxygen) and partially negative (hydrogens). Though the partial charge is less than a full charge, the water molecules can cluster around an ion, or molecule, in solution. In the case of NaCl, the partially positive ends of the polar water molecules cluster around the Cl- ions and the partially negative ends cluster around the Na+ ions, resulting in the formation of Na+ and Cl- ions in solution. This can be either an endothermic (requires heat) or exothermic (gives off heat).