Why do some compounds dissolve and others don't?

*ACHOOOOO*  Well, it has happened again.  You have the watery eyes, the achy head, and the runny nose, which are all the makings of a nice cold.  You had no choice, though.  It is fall and you-know-what is right around the corner. You just had to brave the crowds of people and fight the frigid air to get all your shopping completed.  Nothing you can do about it now, but just wait it out and take some bubbly vitamin-C supplement to ease your symptoms.  So, you get your pint-sized glass of water and mix in your favorite vitamin-C tablet.  As you are stirring the water, you watch the tablet disappear and the water turn into a pleasant yellow-orange color. HA!!  Who do you think you are fooling?  With that cold, you aren’t going out today.  You might as well knock back that bubbly concoction and figure out why some compounds dissolve and others don’t.

To make a long story short, we can say like dissolves in like.  To make a long story, well, long, let’s clear up some vocabulary, first.  When a solid compound dissolves or “disappears” in a liquid, a solution is made.  Within the context of a solution, the solid is called a solute and the liquid is called a solvent.  Temperature, pressure, stirring, and quantity are factors that affect the ability of a solute to dissolve in a solvent.  However, certain solutes don’t dissolve in certain solvents due to differing polarity between the two.  No, by polarity we don’t mean Santa’s North pole versus the South pole. Represent!  Actually, here we are talking about chemical polarity.

Chemical polarity of a molecule describes the distribution of electrical charge amongst the molecule’s atoms and bonds.  Some parts might have a negative charge while the other parts will have a positive charge.  Uneven charge distribution yields a polar solute, while nonpolar solutes have even charge distribution.  Unlike in your favorite romantic-comedy, when it comes to solutions, opposites don’t attract. Polar solutes dissolve in polar solvents, and nonpolar solutes dissolve in nonpolar solvents. A table salt (NaCl, sodium chloride) solute dissolves pretty well in a water (H2O) solvent because both are highly polar substances.  Whereas candle wax (C31H64) doesn’t dissolve in water because wax is nonpolar.  A nice solvent for wax would be hexane (C6H14), just in case you were wondering.

AHHH yes. The vitamin-C is starting to kick-in, and now you have more wisdom about solutions than you ever could have asked for.  Isn’t that stirring? 

By Mark Smith

Question by Eli