Water dissolves salt by dissociating the ions in salt from each other. Because water is a polar molecule, each of its ends holds a slight positive or negative electrical charge. These ends attract the positive and negative ions in salt and pull them apart from each other.
The polarity of water comes from the differences in electronegativity in the atoms involved in the bonding process. When covalently bonded atoms have a difference in electronegativity, the electrons are shared unevenly in the bond and result in opposing slightly negative and positive charged ends. In water, the positive end is composed of the hydrogen atoms and the negative end is the oxygen atom. When interacting with sodium chloride - common table salt - the positive sodium ion is drawn to the oxygen end of water and the negative chloride ion to the hydrogen end.
Although common table salt easily dissolves in water, not all ionic salts do. If the strength of the attraction between the ions is much greater than the strength exerted by the slight charges of the water molecule, the ions remain bonded in water. A set of established rules, known as the solubility rules, provide the general guidelines and exceptions in determining whether an ionic compound or salt is water soluble .