Nonpolar substances tend to be miscible with other nonpolar substances and less miscible with polar substances. At the molecular level the formation of a solution from two nonpolar substances, such as carbon tetrachloride and benzene, can be visualised as a process of simple mixing. The nonpolar molecules, having little tendency to either attract or repel one another, easily intermingle to form a homogeneous mixture. Solution formation between polar substances is much more complex. When an ionic solute is dissolved in a polar solvent, the solvation process commonly occurs.
Solvation is the strong interaction of solute with the solvent molecules.
In the case of water the solvation is called hydration. The importance of this attraction of solvent molecules for the solute in the process of solution is illustrated by the dissolving of sodium chloride in water. In the crystal lattice of sodium chloride, positive sodium ions and negative chloride ions attract each other strongly. Water molecules in contact with a crystal orient themselves to neutralise the attractive forces between the ions in the crystal structure. The positive end of the water dipole is attracted to the chloride ions, and the negative end of the water dipole to the sodium ions.