There are two basic kinds of mixtures, homogeneous and heterogeneous. A homogeneous mixture is composed of a single visible phase while a heterogeneous mixture has two or more visible phases. Aqueous sodium chloride is a homogeneous mixture because NaCl completely dissolves in water, thus giving a single visible phase. The Na+ and Cl- ions are completely dissociated from one another in solution and form electrostatic interactions with hydrogen and oxygen atoms in the water. So the different individual ions are completely intermixed and surrounded by water molecules on the molecular level. This is why the NaCl dissolves and forms a single visible phase to the eye (a homogeneous mixture).