Polarity's Impact on Dissolving

All of us have heard of clichés like that stuck "like water on a duck's back," or know that water and gasoline don't mix.  But why?  Let's briefly review what we investigated during first semester when we first encountered the concept of polarity.

When some molecules are formed, they are electrically neutral due to similar electron affinities of the atoms involved, or to a cancellation effect resulting from the shape of the molecule.  However, some molecules upon forming are not electrically neutral.  One atom, or group of atoms, might have acquired a greater proportion of the electrons being shared.  We would say that this atom or group of atoms had acquired a partial negative charge.  The side of the molecule (or the atom) that has the electron deficit would now be positive.  We would say that this molecule has two ends (a positive and a negative end).  Hence we would say that it has a dipole.  We shorten it to say that the molecule is polar.  Molecules that are electrically neutral are called nonpolar molecules.  See the electrical nature of different bond types page which reviews what we had described previously.  

So what does this have to do with solutions?  Remember that solutions are mixtures of molecules. Let's try to simply picture what would happen if we put together:

• two polar molecules (notice connection between the molecules)
  
  
  

• two nonpolar molecules (notice they can orientate themselves in any way)     
        
  

• a polar and nonpolar molecule (notice that there is no attraction between these molecules either.)

So what happens when we put together many molecules as in solutions or mixtures?  Like we saw when we put together the magnetic marbles with the regular marbles, the two polar and nonpolar molecules separate from one another.  If the nonpolar molecules are less dense than the polar molecules, their interaction might look something like this.

Simply put, polar molecules are attracted to polar molecules.  Nonpolar molecules are just bouncing around.  If this is clear, it is easy to see why people have come up with the phrase describing polarity's effect on solubility

 - Like dissolves Like.  

This implies that

• Polar substances WILL dissolve in polar substances
• Non-polar substances WILL dissolve in nonpolar substances  
• Non-polar substances WILL NOT dissolve in polar substances.  

One term that we will be using often when describing polarity's effect on solutions is the term miscibility.  Miscibility refers to a liquid being mutually soluble in another liquid (implying that they must have the same polarity).  Immiscible means  a liquid not being mutually soluble in the other liquid (implying that the liquids are not of the same polarities).

 This is a little simplistic, but will work for our discussions.  If you care to look into how polarities have an impact on solubility, also look at the concept of induced dipoles.  There is an excellent animation on the web at this location that looks at this issue more closely.