In ionic compounds, at least one atom donates an electron to another atom, bonding the two atoms together like a magnet. If dissolved in a solvent that breaks the ionic bond, the part of the molecule with an additional electron is a negative ion called an anion, whereas the other part of the molecule with fewer electrons is a positive ion, or cation. Water, also referred to as the universal solvent, interacts with ionic compounds and covalent polar compounds.
Water (H₂O) is an oxide of hydrogen and has a pH of seven (neutral). A water molecule is shaped like a mouse's head, with the two hydrogen atoms being "ears" of the oxygen atom. Water is a dipole, meaning it has two poles of electric charge. Positive charges are at one end with the hydrogen, and the negative charge is at the oxygen end. Despite this, water is not ionic. It can dissolve polar substances, and its polarity enables it to dissolve ionic substances.
Water as Solvent
To demonstrate how polar water molecules dissolve compounds, consider hydrogen chloride (HCl) gas. Pure, dry hydrogen chloride gas is covalently bonded (meaning atoms share electrons) and does not ionise. When dissolved in water, however, HCl molecules split up into ions in this fashion:
H₂O + H-Cl ' H₃O⁺ + Cl־
The hydrogen of the gas attaches to the electron-rich back side of the oxygen, forming a hydronium cation, H₃O⁺. This "pulling" process is strong enough to break the hydrogen-chlorine bond.
Ionic compounds dissolve just as readily, since they are fully charged. In the ionic compound sodium chloride, the sodium and chlorine atoms are charged because of an electron transfer from the sodium to the chlorine. The oxygen of water has a slightly negative charge and is drawn to the strong positive charge on sodium. The slightly positive hydrogen atoms of water are drawn to the full negative charge of the chlorine anion. Eventually, the separated sodium and chloride ions are surrounded by water molecules.
Nonpolar, Nonionic Solvents
Nonpolar, nonionic solvents are not drawn to ionic compounds, and, in fact, there is no energy "incentive" for such compounds to dissolve. Energy is to reactions rather like money is to business--it is the driving force for activity.
Certain compounds have molecules that are large in size, being mostly nonpolar and covalent in nature, while having an ionic part at one end. For example, indicating the covalent, nonpolar part with X's, a structure could look like this:
In this case, a petroleum-based substance such as oil may be drawn to the long chain represented here by X's, whereas water is drawn to the sodium cation and the carboxylic acid anion (-COO⁻). All three species (the one drawn above represents a detergent) are thus attached, and the oil is, for all practical purposes, dissolved in water.