Final Exam Review #11, #25

11.  How do you know if a saturated solution has formed?  How do you form a supersaturated solution?  What factors affect solubility and why?

A saturated solution contains the maximum amount of solute, while an unsaturated solution can still dissolve more solute.  Supersaturated solutions contain more solute than theoretically possible at any given temperature when the excess solute doesn't crystallize out of the final solution.  This is done by heating a solution to a higher temperature, which essentially raises the temperature of equilibrium - allowing more solute to be dissolved.  After the solution has cooled, the same amount of solute will still be present in the solution.  The solute will not crystallize until a seed crystal has been dropped into the solution.  Stirring, the size of the solid in the solution, temperature, and the pressure (for gases only) all effect the solubility of a substance.  Stirring helps the solute dissolve faster by moving fresh solvent into contact with the solid solute's surface area at a faster rate.  Smaller crystals expose more surface area, so therefore the smaller the size of the solid, the faster it will dissolve in a liquid solution.  A change in temperature affects how much of the solute is dissolved.  Temperature also affects how fast the solute is dissolved because the kinetic energy of the solvent increases or decreases between the solute and the solvent.  A raise in temperature generally increases the amount of solute than can be dissolved into a solvent.  Higher pressure yields a greater amount of gas dissolved.  For example, opening a bottle or carbonated soda relieves the high pressure and the CO2 escapes due to a decreased solubility (causing the soda to go flat).  Bottles of soda are bottled at high pressures to keep the CO2 dissolved in the coke solution.

25.  How and under what conditions do real gases deviate from ideal behavior?  Which gases would be considered the least ideal?

In ideal gases, the particles theoretically have no volume, and the particles are not attracted to each other.  This means that no solids or liquids would technically form.  No gas is truly ideal in real experiments or life.  This deviation is most obvious at low temperatures and high pressures.  Molecules that are large and/or polar deviate the most from ideal gases because the have large particle volume, and high polarity makes the particles attracted to each other.  Gases do have volume that will resist compression and will form more dense states (like liquids and solids).  This makes them not ideal because the particles of any gas in real life have a volume.  Furthermore, gases do also have intermolecular attractions (such as London-Dispersion Forces and Hydrogen bonds) that allow the gases to form liquids and solids.  Attractions between particles void all gases of being counted as ideal gases.  Gases like radon, xenon, and krypton are considered the least ideal because the are the elemental gases with the largest surface area in terms of their atom size.  This large size of the atom causes the gases to have a theoretical volume, and the particles would also be attracted to each other (polarity) - which is the opposite definition of a theoretical ideal gas.