Solubility of two chemical compounds (solvents) which merge to form one homogeneous blend is determined by the individual chemical properties of the constituent molecules of each solvent. In day to day chores we come across many examples where we mix two liquids or liquids with solids for drinking, cooking, washing etc., least realizing, what makes them into congruent mixtures, retaining specific identities of the comprising compounds. At the same time we also observe that there are some liquids which do not mix with each other and remain distinctly detached when poured into the same container; e.g. edible oil and water. For a student of chemistry it is just another analytical process of the laboratory.
Two solvents or compounds are soluble in each other if they have similar polarities (a variable mix of negative and positive electric charges of their constituent elements). The structural formula and the magnitude of electronegativity of molecules in a solvent resolve the strength of the bond between the molecules of a compound as also the extent of its reactivity with other compounds.
It is pertinent here to visualize the periodic table of elements too. The elements to the right of the table are more electronegative and become lesser so as we move towards the left of the table. In the formation of a compound when the elements of different electric charges (i.e. distantly left and right to each other in the periodic table) combine, they result in molecules having one electric charge, negative or positive, at one end and opposite at the other end of the molecule. So the bonds between atoms with very different electronegativities are created, like a bond between oxygen and hydrogen.
In such cases the molecules of a compound become polar, with negative and positive charges at two ends of molecules. Such compounds are receptive to forming homogeneous mixtures with other polar compounds. It is the electronegative charge of polar solvent molecules which attract oppositely charged molecules to form a homogeneous mixture.
This phenomenon is true only for one group of compounds which are formed with the union of electronegative and not so electronegative elements. Such compounds are known as polar solvents. Another group of compounds is of non-polar solvents which contain bonds between atoms in a molecule with similar electronegativities, such as between carbon and hydrogen (hydrocarbons like gasoline). Bonds between atoms with similar electronegativities lack partial charges. It is this absence of charge which makes these molecules â€œnon-polarâ€. In this set of compounds electric charges within the molecules are evenly scattered and poles are bereft of any particular electric charge, whether negative or positive, making them almost inert to polar solvents.
Hexane (C6H14) is a hydrocarbon in the group alkanes, comprising of six carbon atoms in a straight chain, each tethered to two hydrogen atoms, excepting the first and last carbon atoms of the chain, who are bonded to an extra hydrogen atom each. The constituting elements of hexane, namely carbon and hydrogen have similar electronegativities. Their bonding produces a non-polar effect, i.e. the electronegativity on the poles of its molecules is missing, which is necessary in order to mix with polar solvents like water. Because of absence of this requisite electric charge to attract polar molecules, Hexane becomes non-entertaining to polar solvents. Particularly so, to water. Because of this behavior alkanes, for their failure to mingle with polar solvents, are also nomenclatured Paraffins (para – not enough and affin – affinity or attraction) or saturated (satisfied) hydrocarbons.
As explained earlier in the same context, Hexane is thus a saturated hydrocarbon and like other members of the group is non polar. Meaning thereby that it lacks electronegativity, preventing mixing with polar compounds of which water is a major example.