Nature of solutions

We took a little step away from colligative properties today to make sure we're all familiar with some of the terminology of solutions.  Solubility is not a "yes/no" question.  Is potassium nitrate soluble?  Yes, all potassium salts are soluble and all nitrate salts are soluble, so potassium nitrate must be soluble.  If we take 1 gram of KNO3(s) and add it to 1L of water, it will dissolve.  What if we take 1 kilogram of KNO3(s) and add it to 1mL of water?  Will it dissolve?  Yes, but will ALL of it dissolve?  I think not.  If we start with 1L of water and slowly add KNO3(s), at some point the excess KNO3(s) will no longer dissolve because the solution has become saturated.  A saturated solution represents the maximum amount of a solute that can dissolve in a given amount of solvent.  Sometimes, a solution can become supersaturated when "extra" solute is dissolved; supersaturated solutions are unstable and will form precipitate if a nucleation site is present.  This can be a speck of dust, or a scratch in the glass of the container, or a seed crystal of the substance.

We also looked at the temperature dependence of solubility.  For solids dissolving in liquids, heating the solution will usually allow more solute to dissolve.  This is one way to make supersaturated solutions; a hot saturated solution is allowed to cool in the absence of nucleation sites.  For gases dissolved in liquids, the situation is reversed; cold solvent is usually able to hold more dissolved gas than hot solvent because the gas solute particles in the hot solution have more kinetic energy and are more likely to escape from the solution.

Returning to colligative properties, we began to discuss the most important colligative property in biological systems, osmosis.  When two solutions of differ in concentration are separated by a semipermeable membrane, solvent tends to flow from the less concentrated side to the more concentrated side.  What's a semipermeable membrane?  Cell walls.  Skin.  LOTS of biological things are semipermeable membranes and control function by regulating osmosis.  Awesome.

There's new OWL posted, and don't forget to take the pre-lab quiz before 8:00am tomorrow.